WO2000046964A1

WO2000046964A1 - Telecommunication system and method in a telecommunication system

Info

Publication number: WO2000046964A1
Application number: PCT/SE2000/000205
Authority: WO
Inventors: Johan Rune
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1999-02-02
Filing date: 2000-02-01
Publication date: 2000-08-10
Also published as: SE9900353L; SE9900353D0; SE521089C2; AU2838900A

Abstract

The present invention relates to a Telecommunication System for routing a call from a first subscriber via an originating circuit switch over an IP network via a destination circuit switch to a second subscriber. Today address translation tables for translating standardised addresses used in ISDN and PSTN into IP addresses have to be stored in the IP Telephony Gateways. The administrators of the IP Telephony Gateways have to keep track of the number plans of all the different circuit switched networks. When the network of interconnected IP Telephony Gateways grows the required manual administration of the current solutions increases significantly. This problem is solved by means of a Telecommunication System wherein the local circuit switches store the IP addresses to its associated Gateways.

Description

TELECOMMUNICATION SYSTEM AND METHOD IN A TELECOMMUNICATION

SYSTEM

FIELD OF INVENTION

The present invention relates generally to call routing and particularly to a Telecommunication system for routing a call from a first subscriber via an originating circuit switch over an IP network via a destination circuit switch to a second subscriber.

The invention also relates to a method in the Telecommunication system for routing a call from a first subscriber via an originating circuit switch over an IP network via a destination circuit switch to a second subscriber.

DESCRIPTION OF RELATED ART

Currently the Internet is emerging as a more and more viable alternative to the traditional circuit switched telephone networks like ISDN/PSTN, not only for data traffic, but also for such traditional circuit switched network services as voice and facsimile traffic. The technology is improving and the offered services are becoming increasingly flexible and user friendly in addition to the more attractive prices. Today you do not need any special software, even a computer to be able to use the Internet for telephone calls. You can access the services from almost any phone as well as contact almost any phone. The key to this flexibility is the development and deployment of IP telephony gateways between the traditional circuit switched networks and the Internet. Using a special access number, for a certain IP telephony service provider, a user can get his call routed to the closest IP telephony gateway, then across the Internet to the IP telephony gateway closest to the destination, where the call is converted back to a circuit switched call and routed in the circuit switched network to the destination. In this way the major part of the route, especially for international calls, will be through the Internet instead of the circuit switched network. This is a very advantageous concept, but there are limitations. In the Internet, IP addresses are used for addressing and IP and Internet routing protocols are used for routing. In the circuit switched network on the other hand, standardised addresses like E.163, E.164 or E.212 addresses (i.e. phone numbers) are used for addressing. For call routing in circuit switched network ISUP (ISDN User Part) and TUP (Telephone User Part), of the Common Channel Signalling System No 7 and sometimes older signalling schemes, like e.g. R2 are used. These methods for addressing and routing in the Internet and the circuit switched network are totally different and there is no harmonisation or integration of the two. More specifically, there is no connection between the circuit switched standardised addresses and IP addresses.

This means that there is no obvious way to automatically translate standardised addresses, used in ISDN and PSTN into IP addresses. This in turn means that an IP gateway itself must be able to analyse the received ISDN/PSTN standardised destination address and from the analysis derive the IP address of the correct destination IP telephony gateway. This works fine, however, as long as the connected IP telephony gateways are rather few. When there is, for instance, only one IP telephony gateway in each country, the standardised address analysis is very easy. A simple program and a mapping table translating the country code of a phone number to the IP address of the gateway in the destination country are sufficient.

In WO9836543 is shown a gateway system for processing calls over the Internet. The system uses a distributed database in the Internet to translate the dialled number for a telephone call into a prioritised list of preferred gateways for that call.

However, as the network of IP telephony gateways grows, this solution will not scale well. It will no longer be enough to check only the country codes. The IP telephony gateway must also have knowledge of each destination country's internal number plan in order to be able to route the call to the correct IP telephony gateway. This will eventually create very large address mapping tables, which will not only complicate the standardised address analysis, but will also be very difficult to maintain. Since there is no automatic exchange of address information between the circuit switched network and the Internet, the mapping tables will have to be maintained manually. This does not only mean administering a large amount of data, it also means keeping track of changes in the number plan for all concerned circuit switched network operators in all concerned countries. The tables will also have to be updated when new IP telephony gateways are installed, or when old ones are taken out of service. In a scenario where the popularity of IP based telephony and facsimile traffic increases, generating large volumes of IP based telephone/facsimile traffic, it is obvious that the current solutions for addressing and routing will not scale well. A more automated solution will be needed. Such a solution is what the present invention proposes.

SUMMARY OF THE INVENTION

The present invention deals with the problem of the lack of connection between the circuit switched network address system and the IP address system when routing a call from a first subscriber via an originating circuit switch over an IP network via a destination circuit switch to a second subscriber.

Today address tables for translation from circuit switched network addresses to IP addresses has to be stored in the IP Telephony Gateways or associated databases. The administrators of the IP Telephony Gateways and/or databases have to keep track of the number plans of all the different circuit switched networks. When the network of interconnected IP telephony gateways grows the required manual administration of the current solutions increases significantly.

The object of the present invention is thus, when routing the call, to find out the IP address of the destination switch's closest or for the occation best IP Telephony Gateway by using the inherent circuit switched network routing capabilities.

The aforesaid problem is solved by means of a Telecommunication System wherein the local circuit switches store the IP addresses to their associated Gateway(s). The following scenario of routing the call describes the inventive concept of the present application.

A first subscriber dials a number to a second subscriber. The number is analysed by the originating circuit switch and intermediate switches along the path to the destination circuit switch. By means of this distributed analysis the destination circuit switch is found. The originating circuit switch then interrogates the destination circuit switch of an IP address to its associated IP Telephony Gateway. An associated IP Telephony Gateway can be the closest or for the occasion best IP Telephony Gateway. The IP address is returned to the originating circuit switch and transferred to its associated IP Telephony Gateway. The call can then be routed over the IP network to the second subscriber via the destination IP Telephony Gateway and via one or more subsequent circuit switches.

An advantage of the present invention is that no address translation capability is required in the IP Telephony Gateways or in associated databases.

Another advantage of the present invention is that the administrators of the IP Telephony Gateways do not have to keep track of the numberplans of all the different circuit switched networks.

Another advantage of the present invention is that the manual administration of addresses is reduced to a minimum even for a fast growing network of IP Telephony Gateways.

Yet another advantage is that the present invention offers a way for the traditional circuit switched network operators to get an edge when they diversify their operations into the IP telephony and facsimile business.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the telecommunication system according to the invention.

Figure 2 shows a flowchart over the call routing.

Figure 3 shows a signalling sequence over an indication of an IP network. Figure 4 shows a signalling sequence over an interrogation and return of an

IP address. Figure 5 shows a signalling sequence over an interrogation and return of an

IP address. Figure 6 shows a signalling sequence over an interrogation and return of an IP address.

Figure 7 shows a signalling sequence over an interrogation and return of an

IP address. Figure 8 shows a signalling sequence over an interrogation and return of an

IP address. Figure 9 shows a signalling sequence over an interrogation and return of an

IP address in a tunnel. Figure 10 shows a signalling sequence over an interrogation and return of an

IP address. Figure 11 shows a signalling sequence over a transfer of an IP address. Figure 12 shows a signalling sequence over a transfer of an IP address in a tunnel. Figure 13 shows a signalling sequence over transfer of an IP address.

Figure 14 shows a signalling sequence over the call routing in the case of the circuit switched network being a PSTN. Figure 15 shows a signalling sequence over the call routing in the case of the circuit switched network being an ISDN. DESCRIPTION OF PREFERRED EMBODIMENTS

In figure 1 is depicted the preferred embodiment of the Telecommunication system 101 in which a call can be routed from a first subscriber A in a circuit switched network 103, over an IP network 102 to a second subscriber B in the circuit switched network 103.

Subscriber A has a facsimile 106 or a telephone 107 being connected to an originating circuit switch 104 in the circuit switched network 103, which for example might be an ISDN or a PSTN. Subscriber B has a facsimile 108 or a telephone 109 being connected to a destination circuit switch 105 in the circuit switched network 103. The circuit switches 104 and 105 are connected to each other possibly via one or more intermediate switches 116.

The circuit switched network 103 is connected to the IP network 102 via IP Gateways 110; 113 each IP Gateway having an IP address 112; 115. The originating IP Telephony Gateway 110 has a first IP address 112 and the destination IP Telephony Gateway 113 has a second IP addresses 115. The originating circuit switch 104 is connected to its associated IP Gateway, the originating IP Telephony Gateway 110, possibly via one or more transit switch 111. The destination circuit switch 105 is connected to its associated IP Gateway, the destination IP Telephony Gateway 113 possibly via one or more transit switches 114.

The local circuit switches, which are the circuit switches that can be destination circuit switches, stores the IP address to its associated IP Telephony Gateway. The circuit switch 104 thus stores the IP address 112 to its associated IP Telephony Gateway 110, and the circuit switch 105 stores the IP address 115 to its associated IP Telephony Gateway(s) 113. It is also possible to store IP addresses to alternative IP Telephony Gateways for reliability reasons. Currently there are normally fewer IP Telephony Gateways than circuit switches, but in the future it might be the opposite case.

It is also possible to co-locate the circuit switch and its associated IP Telephony Gateway in one node. The circuit switch 104 and its associated IP telephony Gateway 110 would then be co-located in one node and the circuit switch 105 and its associated IP telephony Gateway 113 would be co-located in one node.

A further possible way is to co-locate the transit switch and its associated IP telephony Gateways in one node. The transit switch 111 and its associated IP telephony Gateway 110 would then be co-located in one node and the transit switch 114 and its associated IP telephony Gateway 113 would be co-located in one node.

Fig. 2 shows a flowchart of a possible scenario of routing a call from a first subscriber A in a circuit switched network 103, over an IP network 102 to a second subscriber B in the circuit switched network 103. The second subscriber B has a B number.

The routing method comprises the following steps:

201 A user A starts with dialling the B number and

202 indicates that the IP network 102 can be used for the concerned call.

203 The B number is partly analysed by the originating circuit switch 104 to begin routing the call and partly analysed by intermediate circuit switch(es) 116 along the route for identifying the destination circuit switch 105.

204 The originating circuit switch 104 interrogates the destination circuit switch 105 for an IP address 115 to its associated IP Telephony Gateway 113. If there is lack of such IP address, the call will be routed over the circuit switched network 103, step 209.

205 The destination switch returns the interrogated IP address 115 to the originating circuit switch 104.

206 The IP address is transferred to and the call is routed to the originating IP Telephony Gateway 110 and

207 the call is routed over the IP network 102,

208 to the B subscriber via the destination IP Telephony Gateway 113 by using the IP address 115 and the B number. Different embodiments of the different steps in the routing method will now be described more in detail followed by a description of the preferred embodiment of the complete method.

A simple way to indicate to the network that an IP network can be used to carry a certain call in step 202 is to dial a predetermined access code before dialling the actual number. Such an access code could be e.g. any number of digits followed by a hash (#) sign followed by the actual subscriber B number. It could also be even simpler, e.g. just push the hash (#) sign before the actual number is dialled.

Another alternative, in the case of PSTN or ISDN is to indicate by default that the IP network 102 can be used for all calls. This can be done implicitly through subscriber data in the local circuit switch 104. If the subscriber A does not want to use the IP network for a particular call he may dial a certain access code before he dials the subscriber B number.

An alternative in the case of ISDN is to indicate the IP network explicitly in the SETUP message from subscriber A as described in Fig. 3. There are a number of ways this can be done, e.g. using a special value in the Transit Network Selection information element. The Transit Network Selection is information sent in the initial address message indicating that the transit network(s) requested to be used in the call as specified in chapter 2.144 in ITU (International Telecommunication Union) recommendation Q.762. A further way is to use a special value in the Network Specific Facilities information element. The Network Specific Facilities is a service- related information, transparently transferred in either direction between the local exchange and the identified network, which contracts the service. The information is significant to both user and the identified network as specified in chapter 2.92 in ITU recommendation Q.762. This information element is used to select the network facilities to be invoked, i.e. selection of the carrier network. A further way is to use a new parameter or a spare bit Bearer Capability information element. A further way is to use one of the reserved values of a so-called User Information Layer three parameter in the Bearer Capability information element. In fig. 4, step 204, in the case of the circuit switched network 103 being an ISDN the interrogation of the IP address can be performed via a so-called ISUP (ISDN User Part) IAM (Initial address message) using the Common Channel Signalling System No 7. IAM is a message sent in the forward direction to initiate seizure of an outgoing circuit and to transmit number and other information relating to the routing and handling of a call as specified in chapter 1.32 in ITU recommendation Q.762.

Within the IAM it is possible to use the optional Remote Operation parameter, which is used to indicate the invocation of a supplementary service identified by an operation value and also carry the result or error indications depending of the outcome of the operation as specified in chapter 2.128 in ITU recommendation Q.762.

Within the IAM it is also possible to use the optional Generic Notification parameter which is information sent in either direction intended to provide supplementary service notification to a user as specified in chapter 2.53 in ITU recommendation Q.762. The destination circuit switch 105 having derived from Generic Notification parameter that the call will not be routed over the circuit switched network 103 can perform the return in step 205 with a REL (Release message) which is a message sent in either direction to indicate that the circuit is being released due to the reason (cause) supplied and is ready to be put into the idle state on receipt of the release complete message. In case the call was forwarded or is to be re-routed, the appropriate indicator is carried in the message together with the redirection address and the redirecting address as specified in chapter 1.37 in ITU recommendation Q.762.

Within the REL it is possible to use the User-to-User Information parameter to carry the IP address 115. This parameter is normally used for information generated by a user and transferred transparently through the interexchange network between the originating and terminating local exchanges as specified in chapter 2.155 in ITU recommendation Q.762. Fig. 5 and Fig. 6 shows that the destination circuit switch 105 also can perform the return of the IP address according to step 205 with an ACM (Address Complete Message) which is a message sent in the backward direction indicating that all the address signals required for routing the call to the called party have been received. As specified in chapter 1.1 in ITU recommendation Q.762.

Within the ACM, again the User-to-User Information or the Remote Operations parameter can be used. If the interrogation was sent in a Remote Operations parameter in the IAM, the address should be returned in a Remote Operations parameter in the ACM. The originating circuit switch 104 then replies with a REL to which the destination circuit switch 105 replies with a RLC (Release Complete Message). See Fig. 5. A RLC is a message sent in either direction in response to the receipt of a released message, or if appropriate to a reset circuit message, when the circuit concerned has been brought into the idle condition as specified in chapter 1.38 in ITU recommendation Q.762.

As an alternative the originating circuit switch 104 can send a RLC as response of the ACM. See Fig. 6.

In fig 7 are other alternatives within step 204 shown. In the case of the circuit switched network 103 being an ISDN, it is possible to perform the interrogation via a particularly designed ISUP message, a particularly designed optional parameter or particularly designed optional parameter value in an ISUP message.

Other alternatives within step 205, in the case of the circuit switched network 103 being an ISDN, are to perform the return of the IP address 115 via a particularly designed ISUP message or via a particularly designed optional parameter in an ISUP message, also shown in fig. 7.

When a new message or a parameter is introduced or a parameter which is already in use for other purposes is used in step 204 or 205, an instruction indicator must be included to instruct the intermediate circuit switch(es) 116 to pass along the message or parameter even if it is not recognised, see fig.8. The instruction indicators are included in the two parameters Message Compatibility Information parameter and Parameter Compatibility Information parameter.

The Message Compatibility Information Parameter relates to information sent in either direction indicating how an exchange should react in case this message is 5 unrecognised as specified in chapter 2.79 in ITU recommendation Q.762. Within the Message Compatibility Information Parameter the Discard Message Indicator should be set to PASS ON. The Discard Message Indicator is information sent to inform another node whether to discard the related message, due to compatibility reasons unrecognised as specified in chapter 2.40 in ITU recommendation Q.762. The l o Discard Message Indicator is also present in the Parameter Compatibility Information Parameter.

In the case of a particularly designed optional parameter or particularly designed optional parameter value the Parameter Compatibility Information Parameter should used which relates to a piece of information sent in either direction indicating how an

15 exchange should react in case the parameter is unrecognised as specified in chapter 2.105 in ITU recommendation Q.762. Within the Parameter Compatibility Information Parameter, the Discard Parameter Indicator should be set to PASS ON. The Discard Parameter Indicator is information sent to inform another node to discard the related parameter, due to compatibility reasons unrecognised as

20 specified in chapter 2.41 in ITU recommendation Q.762.

In fig. 9, an alternative to the instruction indicators mentioned above is shown. In case of intermediate switches 116 and in the case of ISDN it is possible to use the APM (Application Transport Mechanism) of ISUP to create a tunnel 117 between the originating circuit switch 104 and the destination circuit switch 105 via the 25 intermediate switch(es) 116. APM is an addition to ISUP which provides a bearer for application specific signalling, as specified in ITU recommendation ITU recommendation Q.765. This bearer can be thought of as a 'tunnel', since the information transferred between the APM user applications is transparent to the intermediate nodes.

30 In the case of the circuit switched network 103 being a PSTN, the interrogation according to step 204 can be performed via a spare bit in the IAM (Initial Address Message) of TUP (Telephone User Part),as described in fig. 10. Initial Address Message is a type of message sent first in the forward direction at call set-up. It contains address information and other information relating to the routing and handling of the call. As specified in chapter 1.1.1 in ITU recommendation Q.722. It is also possible to particularly design an optional parameter in the TUP message or particularly design a TUP message for the interrogation.

In fig. 10 is also described an alternative of returning the IP address 115 in step 205 and in the case of PSTN. In this case it is possible to particularly design a TUP message.

Fig. 11 describes one embodiment of transferring of the second IP address 115 from originating circuit switch 104 to the originating IP Telephony Gateway 110, step 206. The transferring is performed via a particularly designed ISUP (ISDN User Part) or TUP (Telephone User Part) message.

The transferring according to step 206 can be performed via internal communication means if the IP Telephony Gateway 110 is located in the same node as either the associated circuit switch 104 or the transit switch 111.

In the same step 206 there are other alternatives if the circuit switched network is an ISDN. One, shown in fig. 12. is to transfer the second IP address 115 transparent via a specifically designed tunnel 118 between the circuit switch 104 and the appropriate IP Telephony Gateway 110 by means of the APM (Application Transport Mechanism) of ISUP.

Three other alternatives are to perform the transfer via the ISUP by means of the Generic Number parameter or Generic Reference parameter or the User-to-User Information parameter of the IAM (Initial Address Message), see fig. 13.

For the same step, 206 but in the case of the circuit switched network 103 being a PSTN the transferring can be performed via a specifically designed parameter in the IAM (Initial Address Message) of TUP (Telephone User Part). Preferred embodiments of the complete method, one for PSTN and one for ISDN will now be described, the different steps taken from the description above.

First the case of PSTN which is shown in Fig. 14. Subscriber A dials 201 the subscriber B number. Subscriber data in the circuit switch 104 indicates 202 that using an IP network for the call routing is default alternative, this can be overridden by subscriber A by using a special access code. The B number is partly analysed 203 by the originating circuit switch 104 to begin routing the call and partly analysed by intermediate circuit switch(es) 116 along the route for identifying the destination circuit switch 105. The originating circuit switch 104 then interrogates 204 the destination circuit switch 105 of the IP address 115 of its associated IP Telephony Gateway 113 by means of one of the spare bits in the IAM message. After returning 205 the IP address 115 in a particularly designed TUP message, the IP address 115 is transferred 206 in a particularly designed parameter in the IAM to the originating IP Telephony Gateway 110. the call is then routed 207 over the IP network 102 to the destination Telephony Gateway 102 using the IP address 115 and further routed 208 to the B subscriber in the PSTN network 103 via the destination circuit switch 105 using the B number.

The preferred embodiment of ISDN is shown in Fig. 15. Subscriber A dials 201 the subscriber B number. The SETUP message indicates 202 the use of the IP network for the call by means of the Transit Network Selection information element. The B number is partly analysed 203 by the originating circuit switch 104 to begin routing the call and partly analysed by intermediate circuit switch(es) 116 along the route for identifying the destination circuit switch 105. The originating circuit switch 104 then interrogates 204 the destination circuit switch 105 for the IP address 115 of its associated IP Telephony Gateway 113 using the Remote Operations parameter of the IAM message. After returning the IP address 115 in a Remote Operations parameter of the ACM message, the IP address 115 is transferred 206 by means of one unused value of the Generic Number parameter in the IAM to the originating IP Telephony Gateway 110. the call is then routed 207 over the IP network 102, and further routed 208 to the B subscriber in the PSTN network 103 via the destination circuit switch 105 using the B number. As yet a variation of the embodiments the destination circuit switch can return one, two or more IP addresses to one, two or more corresponding IP Telephony Gateways in order to provide redundancy. All the IP addresses are also transferred from the originating circuit switch to the originating IP Telephony Gateway. If more than one IP address is returned, the first one in the list belongs to the (for this specific call) preferred destination IP Telephony Gateway. However, if the preferred destination IP Telephony Gateway is currently not reachable from the originating IP Telephony Gateway, or otherwise out of operation, the second IP address in the list is used. If no success is achieved with the second IP address, the next one in the list is tried, until the call is successfully routed across the IP network to a destination IP Telephony Gateway or the list is finished.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A Telecommunication system (101 ) for routing a call from a first subsCTiber (A) over an IP network (102) to a second subscriber (B), the Telecommunication system (101) comprising

a circuit switched network (103) including

an originating circuit switch (104) and a destination circuit switch (105),

the originating circuit switch (104) being connected to a first subscriber line (106; 107) for the first subscriber (A),

the destination circuit switch (105) being connected to a second subscriber line

(108;109) for the second subscriber (B),

the originating circuit switch (104) being associated with an originating IP Telephony Gateway (110),

the originating IP Telephony Gateway (110) having a first IP address (112),

the destination circuit switch (105) being associated with a destination IP

Telephony Gateway (113),

the destination IP Telephony Gateway (113) having a second IP address (115),

the IP Gateways (110; 113) each having an interface to the IP network (102),

characterized in that each of the circuit switches (104; 105) stores an IP address to its associated IP Telephony Gateway(s) (110; 113) for routing the call from the first subscriber (A) via the originating circuit switch (104) over the IP network (102) via the destination circuit switch (105) to the second subscriber (B).

2. A Telecommunication system according to claim 1 characterized in that the circuit switch (104; 105) and its associated IP Telephony Gateway (110; 113) is co-located in one node.

3. Telecommunication system according to claim 1 characterized in that the circuit switched network (103) includes one or more intermediate switches (116) between the originating circuit switch (104) and the destination circuit switch (105).

4. Telecommunication system according to claim 3 characterized in that IP Telephony Gateway (110,113) is co-located in one node with an associated transit switch (111,114).

5. Method for routing a call in a Telecommunication system (101 ) according to any of claims 1-4, from a first subscriber (A) via an originating circuit switch (104) over an IP network (102) via a destination circuit switch (105) to a second subscriber (B)

characterized in comprising the steps of

a) dialling (201 ) a subscriber (B) number;

b) indicating (202) that the IP network (102) can be used for the call;

c) analysing (203) the dialled subscriber (B) number for identifying the destination circuit switch (105);

d) interrogating (204) the destination circuit switch (105) for the second IP address (115) to its associated destination IP Telephony Gateway (113);

e) returning (205) the second IP address (115) to the originating circuit switch (104);

f) transferring (206) the second IP address (115) to originating IP Telephony Gateway (110) and routing the call to the originating IP Telephony Gateway (110);

g) routing (207) the call to the destination IP Telephony Gateway (113), over the IP network (102), using the second IP address (115);

h) routing (208) the call to the second subscriber (B) via the destination circuit switch (105).

6. Method according to claim 5, characterized in that in the step b) (202) indicating that the IP network (102) can be used for routing the call by dialling a predetermined access code before dialling the subscriber (B) number.

7. Method according to claim 5, characterized in that in the step b) (202) indicating implicitly through subscriber data that the IP network (102) can be used for routing the call.

8. Method according to claim 7, characterized by dialling a certain access code before dialling the subscriber (B) number indicating that this particular call will not be routed over the IP network.

9. Method according to claim 5, characterized in that in the step b) (202), in the case of the circuit switched network (103) being an ISDN, indicating explicitly in a so-called SETUP message from the (A) subscriber that the IP network (102) can be used for routing the call.

10. Method according to claim 9 characterized in performing the indication in the so- called SETUP message via a special value in the Transit Network Selection information element.

11. Method according to claim 9 characterized in performing the indication in the so- called SETUP message via a special value in the Network Specific Facilities information element.

12. Method according to claim 9 characterized in performing the indication in the so- called SETUP message via a new parameter in the Bearer Capability information element.

13. Method according to claim 9 characterized in performing the indication in the so- called SETUP message via a spare bit in the Bearer Capability information element.

14. Method according to claim 9 characterized in performing the indication in the so- called SETUP message via one of the reserved values of a so-called User

Information Layer 3 parameter in the Bearer Capability information element.

15. Method according to claim 5, characterized in that in the step d) (204) and in the case of the circuit switched network (103) being an ISDN, performing the interrogation via a so-called ISUP (ISDN User Part) IAM (Initial Address

Message).

16. Method according to claim 15, characterized by interrogating the second IP address by means of the optional Remote Operations parameter of the IAM.

17. Method according to claim 15, characterized by interrogating the Second IP address by means of the optional Generic Notification parameter of the IAM.

18. Method according to claim 15, characterized in that in the step d) (204), the destination circuit switch (105) derives from Generic Notification parameter that the call will not be routed over the circuit switched network (103).

19. Method according to claim 17 or 18, characterized in that in the step e) (205) performing the return via a so-called ISUP REL (Release) message using the User-to-User Information parameter.

20. Method according to any of the claims 15-18, characterized in that in the step e) (205), performing the return via an ACM (Address Complete Message)

21. Method according to claim 20 characterized by returning the Second IP address by means of the optional User-to-User Information parameter of the ACM.

22. Method according to claim 20 characterized by returning the Second IP address by means of the optional Remote Operations parameter of the ACM.

23. Method according to any of the claims 15-22, characterized by the originating circuit switch (104) replying with a REL (Release) message.

24. Method according to any of the claims 15-22, characterized by the originating circuit switch (104) replying with a RLC (Release Complete) message.

25. Method according to claim 5, characterized in that in the step d) (204) and in the case of the circuit switched network (103) being an ISDN, performing the interrogation via a particularly designed ISUP (ISDN User Part) message.

26. Method according to any of the claims 5 or 25, characterized in that in the step e) (205) and in the case of the circuit switched network (103) being an ISDN, performing the return via a particularly designed ISUP (ISDN User Part) message.

27. Method according to claim 5, characterized in that in the step d) (204) and in the case of the circuit switched network (103) being an ISDN, performing the interrogation via a particularly designed optional parameter in an ISUP (ISDN User Part) message.

28. Method according to any of the claims 5 or 27, characterized in that in the step e) (205) and in the case of the circuit switched network (103) being an ISDN, performing the return via a particularly designed optional parameter in an ISUP (ISDN User Part) message.

29. Method according to claim 5, characterized in that in the step d) (204) and in the case of the circuit switched network (103) being an ISDN, performing the interrogation via a particularly designed optional parameter value in an ISUP (ISDN User Part) message.

30. Method according to any of the claims 16, 18, 19, 25, 27 or 29 characterized in that in the step d) (204) and in the case of one or more intermediate switch(es) (116) further comprising the step of including an instruction parameter for instructing the intermediate exchanges to pass on the second IP address interrogation unchanged.

31. Method according to any of the claims 26 or 28, characterized in that in the step e) (205) and in the case of one or more intermediate switch(es) (116) further comprising the step of including an instruction indicator for instructing the intermediate exchanges to pass on the second IP address 115 return unchanged.

32. Method according to claim 30 or 31 , characterized in the case of a particularly designed ISUP (ISDN User Part) message, in the Message Compatibility Information parameter setting the Discard Message Indicator to PASS ON.

33. Method according to claim 30 or 31, characterized in the case of a particularly designed optional parameter or parameter value in an ISUP message, in the Parameter Compatibility Information parameter setting the Discard Parameter

Indicator to PASS ON.

34. Method according to claim 5, characterized in that in the step d) (204) and e) (205), and in the case of the circuit switched network (103) being an ISDN, using Application Transport Mechanism (APM) of ISUP to create a tunnel between the originating circuit switch (104) and the destination circuit switch (105) via the intermediate switch(es) (116) for transparently transferring second IP address interrogation and return messages.

35. Method according to claim 5, characterized in that in the step d) (204) and in the case of the circuit switched network (103) being a PSTN, performing the interrogation via a spare bit in the IAM (Initial Address Message) TUP (Telephone User Part) message.

36. Method according to claim 5, characterized in that in the step d) (204) and in the case of the circuit switched network (103) being a PSTN, performing the interrogation via a particularly designed optional parameter in the TUP (Telephone User Part) message.

37. Method according to claim 5, characterized in that according to step d) (204) and in the case of the circuit switched network (103) being a PSTN, performing the interrogation via a particularly designed TUP (Telephone User Part) message.

38. Method according to any of the claims 35-37, characterized in that according to step e), performing the return via a particularly designed TUP message.

39. Method according to any of the claims 35-37, characterized in that according to step e), performing the return via a particularly designed parameter in the so- called TUP ACM (Initial Address Message).

40. Method according to claim 5, characterized in that in the step f) (206), performing the transferring via a particularly designed ISUP (ISDN User Part) or TUP (Telephone User Part) message.

41. Method according to claim 5, characterized in that in the step f) (206) and in the case that the IP Telephony Gateway(s) (110; 113) and appropriate circuit switch(es) (104,105) are located in the same node, performing the transferring via an internal communications means.

42. Method according to claim 5 when depending on claim 4, characterized in that in the step f) (206)and in the case that the transit switches (111;114) and appropriate IP Telephony Gateway(s) (110;113) are located in the same node, performing the transferring via an internal communications means.

43. Method according to claim 5, characterized in that in the step f) (206), and in the case of the circuit switched network (103) being an ISDN, transferring the second IP address (115) transparently via a specifically designed tunnel between the originating circuit switch (104) and the associated IP Telephony Gateway (110) by means of the Application Transport Mechanism of ISUP (ISDN User Part).

44. Method according to claim 5, characterized in that in the step f) (206), in the case of the circuit switched network (103) being an ISDN, performing the transfer of the second IP address (115) via the ISUP (ISDN User Part) IAM (Initial Address l o Message).

45. Method according to claim 44, characterized by transferring the second IP address (115) by means of the Generic Number parameter of the IAM.

15 46. Method according to claim 44, characterized by transferring the second IP address (115) by means of the Generic Reference parameter of the IAM.

47. Method according to claim 44, characterized by transferring the second IP address (115) by means of the User-to-User Information parameter of the IAM.

20

48. Method according to claim 5, characterized in that in the step f) (206), in the case of the circuit switched network (103) being an PSTN, performing the transferring of the second IP address (115) to the destination IP Telephony Gateway (113) via a specifically designed parameter in the TUP (Telephone User Part) IAM (Initial

25 Address Message).