JPH10336287A - Digital telephone set - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a digital telephone set that is provided with a function where a telephone number is entered by the operation of a person in charge and an investigation command is issued to process number investigation automatically and quickly without troubling an opposite party and an invalid telephone number is surely recognized. SOLUTION: When a ten-key is used to enter a telephone number after depressing an investigation key 2a in an operation entry section 2 of the digital telephone set 1, an operation mode of 'number investigation' is displayed on a display device 3, the entered telephone number is displayed, and the number investigation processing is immediately started. The number of the investigation object is used for an incoming call number and a 'setup' message where non-limit digital information is designated as a transmission capability is sent to the network. In the case that the call is not accepted and an 'interrupt' message is transferred from the network, an interruption restoration sequence is executed immediately. The content of the reason display of the information element from the network is analyzed, and whether the telephone number is validated or invalidated is discriminated and the result is displayed on the display device 3. When the content of the reason display indicates a 'missing number', an invalidated number is displayed on the display device 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital telephone connected to an ISDN basic interface,
In particular, the present invention relates to a digital telephone provided with a function for easily and quickly checking the validity of a telephone number in addition to a normal telephone call function.

[0002]

2. Description of the Related Art When a contract is made with a customer for a credit sale or a credit transaction, the customer's credit is investigated in various forms. As a simple and representative credit check, a method is often used in which basic personal information such as address, name, telephone number, date of birth, gender, etc. is declared, and a call is made to the telephone number to confirm it. I have.

[0003]

After the customer has completed the contract procedure, after an appropriate period, a call is made to the declared telephone number, and a clerk conducts a credit check by talking with the other party. This is certainly an effective means of detecting fraudulent contracts. However, as part of the contact process for contracting with a customer, as a way to first confirm that the phone number of the customer who is currently contracting is not a random number (unused number), Calling to is not appropriate.

[0004] Depending on the contents of the credit sale or the transaction, the customer first declares a telephone number, immediately checks whether the number is valid or invalid, and if the invalid telephone number is declared, at that time In some cases, it is desirable to proceed with the business, such as canceling the contract procedure. There were no convenient devices or systems for such purposes.

An object of the present invention is to operate a staff member to input a telephone number and issue a search command, thereby automatically and quickly performing a number search process without inconveniencing the other party. It is an object of the present invention to provide a digital telephone having a function for surely knowing the telephone number of the digital telephone.

[0006]

A digital telephone according to the present invention is connected to a basic interface of an ISDN, and includes a handset, a numeric key for inputting a telephone number and the like, and a mode selection key for designating an operation mode. , An indicator for displaying the telephone number entered in this operation input unit and the results of the number survey described below, and for talking with the other party of the telephone number entered in the call mode And a telephone number processing means for executing the following processing (1), (2) and (3) for the telephone number input in the research mode.

(1) In accordance with the circuit switching call control procedure specified in ITU-T Recommendation Q.931, the telephone number to be investigated is set as the called number, and unrestricted digital information or restricted digital information is specified as the transmission capability. Send a "call setup" message to the network.

(2) When a call of the transmitted "call setting" message is accepted and a "call" message or a "response" message is transferred from the network, a "disconnection" message is immediately transmitted to the network to disconnect the network. A restoration sequence is executed, and the fact that the telephone number is valid is displayed on the display.

(3) If a "disconnect" message is transferred from the network without receiving the call of the transmitted "call setting" message, a disconnection recovery sequence is immediately executed and a "disconnect" message from the network is executed. The contents of the reason display of the information element attached to the telephone number are analyzed to determine whether the telephone number is valid or invalid, and the result of the determination is displayed on the display. When the content of the reason display is "missing number", the fact that the telephone number is invalid is displayed on the display.

In the second invention, in addition to the above requirements (1), (2) and (3), the number checking means is attached to the "disconnection" message transferred from the network. Analyze the reason display of the information element, and if the content of the reason display is "changed subscriber number", obtain the new telephone number entered in the diagnostic information field of the reason display, and Is displayed on the display together with the new telephone number.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION

=== Basic Configuration and Operation of System === FIG. 1 shows a functional configuration of a circuit of a digital telephone according to an embodiment of the present invention, and FIG. 2 shows an external appearance example of the telephone. The digital telephone 1 is connected to a line connection device (DS
U) 30 to the ISDN 40. As is well known, the ISDN 40 and the telephone network 50 are integrally interconnected. As shown in FIG. 2, the appearance of the telephone 1 is not particularly different from ordinary telephones. The telephone body is provided with an operation input unit 2 including numeric keys and various function keys, a display 3 composed of a liquid crystal display panel, an LED lamp attached to the keys, and a handset 4 set. Is done.

A line interface 5 is connected to the home extension from the DSU 30. The line interface 5 includes a driver circuit for transmitting a signal to the network side via the ISDN basic interface, and a receiver circuit for receiving a signal from the network side via the basic interface. The protocol processing unit 6 performs layer 1 processing of the ISDN user / network interface and layer 2 processing relating to the D channel for the layer 1 frame signal exchanged with the line interface 5. The communication circuit 7 connects the protocol processing unit 6 and the handset 4 to perform input / output processing of an audio signal accompanying a call. The central processing unit 8 is composed of a microcomputer, processes key input signals from the operation input unit 2, processes for displaying various types of information using the display 3 and LED lamps, and processes the layer 3 line switching control procedure. Processing as the number search means of the present invention and the telephone 1
It performs the overall sequence control of.

In the same manner as an ordinary digital telephone, receiving a telephone number input from the operation input unit 2, executes protocol processing for a telephone call as a calling terminal, or responds to an incoming call from a network to perform a telephone call. Is performed as a called terminal. The present invention does not propose a new technique for the call function, and thus the description thereof is omitted. A feature of the present invention resides in that the following number search function is added to a digital telephone having a normal telephone call function.

=== Operation Sequence Introducing Unit for Number Checking Process === As an example, let us proceed assuming the following usage form. A digital telephone 1 of the present invention is installed at a window for conducting some kind of credit transaction with a customer, and a clerk handles this telephone 1. The customer fills out a predetermined application form and gives it to the staff. The clerk performs various paperwork according to the entries on the received application form, one of which is "investigation of telephone number". This number search is a check for confirming that the telephone number declared by the customer is not an unused and invalid telephone number. In other words, if you have declared an invalid telephone number, this is a survey to find out immediately.

In the digital telephone 1 shown in FIG.
After pressing the survey key 2a included in the operation input unit 2, the telephone number is input using the numeric keypad. Then, the operation mode of “number search” is displayed on the display 3 and the input telephone number is displayed, and the number search process is started immediately.

FIG. 3 is a flowchart showing the procedure of the number check process. First, a telephone number input as a survey target is acquired, and a call setting sequence is started (step 100 → 200). In the call setting sequence, first, a "call setting" message in which the telephone number to be investigated is the called number and unrestricted digital information is specified as the transmission capability is created and transmitted to the network (step 201). This call setup sequence 200 proceeds according to the circuit-switched call control procedures specified in detail in ITU-T Recommendation Q.931. Since the control procedure of the circuit-switched call is described in detail in various documents, it will not be described in detail in this specification, but a typical sequence proceeds as follows.

The network which has received the "call setting" message from the calling terminal sends a "call setting accepted" message notifying the B channel to be used to the calling terminal, and sends a "call setting" message to the called terminal. With this, various performances required of the receiving terminal are specified. The receiving terminal checks the performance, and the receiving terminal that has confirmed conformity returns a "call" message to the network (the receiving terminal is in a calling state). This "call" message is sent from the network to the originating terminal. When a response such as off-hook is made at the receiving terminal, a "response" message is sent from the receiving terminal to the network to the calling terminal, and a "response confirmation" message is sent from the calling terminal to the network and from the network to the receiving terminal. Can be Thus, the "call setting" message is accepted and the line is connected.

Further, there are cases where a call requested by a calling terminal cannot be accepted for various reasons. In that case, the network sends a "disconnect" message to the originating terminal and performs a disconnect recovery sequence. At this time, in the reason display item of the information element attached to the "disconnect" message sent from the network to the calling terminal, the reason why the call is not accepted is notified in the form of a reason number.

=== Reason indication of "disconnect" message ==
= In ITU-T Recommendation Q.931, the type and number of the reason display attached to the "disconnection" message are defined as follows. 1. Normal class << Reason number 1 missing number >> This reason display indicates that the number was valid in format, but cannot be reached at the destination requested by the calling user because the number is currently missing. << Reason No. 2 No route to the designated transit network >> This cause indicates that the source device of the cause has received a request to route the call through a special transit network which cannot be recognized. The source device of this cause is either a non-existent transit network or, if present, a special transit network that does not service the destination device of this cause. Therefore, it does not recognize the relay network. << Reason number 3 No route to the other party >>
Indicates that the network to which the call was routed could not deliver the call to the called user because it did not receive the call request. << Reason No. 6 Channel Unavailable >> This reason display indicates that the channel selected as a result of the channel selection is unavailable to the transmitting terminal. << Reason number 7 Incoming call to channel where call has been set >> This reason display indicates that the user has received a call, and the call has already been set as a similar call (for example, an X.25 virtual call in packet mode) Indicates that it is connected to the current channel. << Reason No. 16 Normally Disconnected >> This reason display indicates that the call is disconnected when one of the users involved in the call requests a call disconnection recovery. Under normal conditions,
The source of this cause is not the net. << Reason No. 17 Received User Busy >> This reason display indicates that the called user cannot accept another call. In this case, the user equipment must ensure communication integrity for incoming calls. << Reason No. 18 No user response received >> This cause is indicated by the other party's assigned time specified for the call setup message (timer T303 defined in this specification,
Or, when a message indicating “calling” or “answer” is not returned within T310 expiration). << Reason No. 19 Calling user called / no response >> This reason display is used when a call is notified but no response is notified within a specified time thereafter. This cause is not necessarily caused by the Q.931 procedure, but may be caused by a timer in the network. << Reason No. 20 No Subscriber >> This cause indicates that the mobile station has been logged off (by a signaling procedure over the wireless bus) or that radio communication cannot be performed with the terminating mobile station (e.g., failure, out of service, or power loss). ). << Reason No. 21 Rejection of Communication >> This reason display is used to indicate that the device that transmitted this cause display is neither busy nor inconsistent, and can accept the call, but does not accept the call. << Reason No. 22 Change of the other party's subscriber number >> This reason display is to notify the calling user when the called number indicated by the calling user is no longer assigned. At this time, the new called number is included in the diagnostic information field. << Reason No. 26 Disconnection recovery of unselected user >>
This cause indicates that the user did not get the incoming call. << Reason No. 27 Failure of partner terminal >> This reason display indicates that an incoming call to the partner user requested by the calling user could not be performed because the receiving side interface was not functionally normal. This “function is not normal” means that the signaling message does not reach the partner user. For example, a failure of the physical layer or the data link layer of the partner user interface, or a case where the user device is offline. << Reason No. 28 Invalid Number Format (Incomplete Number) >> This reason display indicates that an incoming call cannot be made to the other user because the incoming number is in an invalid format or incomplete. << Reason number 29 Facility rejected >>
Returns if the facility requested by the user cannot be provided on the network. << Reason number 30 Response to status inquiry >>
The "status indication" message is included in the "status indication" message if the reason for generating the "status indication" message is due to the fact that the "status inquiry" message was received before the generation. << Reason No. 31 Other Normal Class >> This reason display is used only for reporting a normal status when it cannot be expressed for other reasons in the normal class. 2. Resource Unavailable Class << Reason Number 34 No Available Line / Channel >> This cause indicates that there is currently no suitable line / channel available to handle the call. << Reason No. 38 Network Failure >> This reason display indicates that the network is not functionally normal and the state is likely to last for a relatively long time. For example, there is a case where it is unlikely to be accepted even if the call is immediately re-issued. << Reason No. 41 Temporary Failure >> This reason display indicates that the network is not functionally normal and the state is unlikely to continue for a long time. For example, there is a case where the user can request a re-call almost immediately. << Reason No. 42 Switching Unit Congestion >> This reason indication is used to indicate that the switching apparatus causing this reason is congested with high traffic. << Reason number 43 Access information discard >>
This indicates that the network could not deliver the requested access information (user / user information, low-layer compatibility, high-layer compatibility, or subaddress indicated in the detailed information) to the other user. A special type of discarded access information may be included in the diagnostic information. << Reason No. 44 Requested Line / Channel Unavailable >> This reason display is returned when the line or channel notified by the requesting terminal cannot be provided by the interface on the partner side. << Reason No. 47 Other Resource Unavailable Class >> This reason indication is used to report as a network congestion event only when another reason in the network congestion class cannot be applied. 3. Service unavailable class << reason number 49 QOS unavailable >> This cause is used to report that the requested QOS (as defined in Recommendation X.213) is not provided (eg, throughput or propagation delay). Is not provided). << Reason No. 50 Requested Facility Not Contracted >> This reason indication indicates that the required supplementary service is not provided by the network because the user has not completed the necessary administrative procedures for the network providing the facility. << Reason No. 57 Transmission Capability Not Permitted >> This reason display is a transmission capability realized by the device that generated the reason display, but indicates that the user has requested a transmission capability not permitted to the user. << Reason No. 58 Currently unavailable transmission capability >> This reason display is a transmission capability realized by the device that generated the reason display, but indicates that the user has requested a transmission capability that is currently unavailable. << Reason No. 63 Unavailable Class of Other Service or Option >> This cause is used to report a service unavailable event only when other cause indications of the service unavailable class cannot be applied. 4. Service not provided class << reason number 65 unprovided transmission capability designation >> This reason indication indicates that the device which transmitted the cause indication does not support the requested transmission capability. << Reason No. 66 Unprovided Channel Type Designation >> This reason display indicates that the device that transmitted the cause display does not support the requested channel type. << Reason No. 69 Unprovided Facility Request >> This cause indicates that the device that transmitted this cause does not provide the requested supplementary service. << Reason No. 70 Restricted digital information transmission capability >> When a certain device requests the unrestricted digital information transmission capability, the device transmitting this cause indicates only the restricted digital information transmission capability among the requested transmission capabilities. Indicates that it is supported. << Reason No. 79 Other Service or Optional Unprovided Class >> This cause is used to report the non-provided service only when no other reason of the unserved class is applicable. 5. Invalid message class << Reason number 81 Invalid call number used >> This cause indicates that the device that transmitted the cause display has received a message with a call number different from the one currently used between the user and the network interface. << Use of Reason Number 82 Invalid Channel Number >> This reason display indicates that the device that has transmitted the cause display has received a request to use a channel number that has not been activated. For example, even though the user has subscribed to channels 1 to 12, the user equipment or the network has channel numbers 13 to 2
3 is attempted. << Reason number 83 No designated suspended call identification number is used >> This reason indication indicates that an attempt has been made to resume a call having a call identification number different from the call identification number used for the suspended call. . << Reason No. 84 Suspended call identification in use >> This cause indicates that the network has received a request to suspend the call, but this suspension request is already used as a suspended call identification in the interface area where call resumption may take place. Indicates that a call identification (including the case where nothing is performed) was included. << Reason No. 85 No suspended call >> This cause indicates that the network has received a request to resume the call. However, the call identification included in the call resumption request indicates that it is not currently being used for a suspended call in the interface area where call resumption may occur. << Reason No. 86 Discontinued call with designated interruption has been restored >> This cause indicates that the network has received a resume request, but this resume request includes the call identification once notified as a suspended call, and that the suspended Indicates that disconnection has already been recovered (by user disconnection or timeout). << Reason No. 87 User is not a CUG member >> Refer to supplementary service specifications << Reason No. 88 Terminal attribute mismatch >> This cause is indicated by lower layer compatibility, higher layer compatibility, or other factors that the device that sent the cause indication cannot conform. Consistency attribute (example =
(Data rate). << Reason No. 91 Invalid Relay Network Selection >> This reason display indicates that the relay network weir has been received, but is in an incorrect format, as defined separately. << Reason No. 95 Other Invalid Message Class >> This reason display is used to report an invalid message class event only when other reasons of the invalid message class cannot be applied. 6. Procedure error (eg, unrecognized message) class << reason number 96 required information element shortage >>
This indicates that the information element (essential information element) that must be present in the message was insufficient when the device that transmitted the reason indication received the message. << Reason number 97 Message type not defined or not provided >>
This cause indicates that the device that sent the cause has received an undefined message or a message with an unrecognizable message type that is defined but not provided at that device. << Reason No. 98 Message mismatch with call state or message type undefined >> This cause indicates whether the message transmitted by the device that transmitted this cause during the particular call state is not an acceptable message during processing. Indicates that a "status display" indicating a call state that has not been performed has been received. << Reason No. 99 Information element undefined >> This reason indication is used when a message including an information element which is undefined even if the information element identifier of the device which transmitted the cause indication is not defined or is defined is received. . It should be noted that the information element is limited to a case where the device which has transmitted the reason is not necessary for processing the message. << Reason number 100 Information element content invalid >> This cause is an information element provided by the device that transmitted the cause, but one or more fields of the information element are codes that are not provided. Is received. << Reason number 101 Call state and message mismatch >> This reason display is used when the received message does not match the call state. << Reason No. 102 Recovery by Timer Expiration >> This reason display indicates that the procedure has been started due to the expiration of the timer according to the error processing procedure of the present Layer 3 specification. << Reason number 111 Other procedure error class >> This reason indication is used to report a procedure error event only when other reasons of the procedure error class cannot be applied. 7. Interworker class << Reason number 127 Other interworking classes >>
This cause indicates that interworking has occurred in the network where there is no cause to operate. Thus, the exact cause for the message to be sent cannot be ascertained.

=== Core of Operation Sequence of Number Checking Process === In the telephone number checking process of the digital telephone 1 of the present invention, as shown in the flowchart of FIG.
In the call setting sequence 200, the “call setting” transmitted
When a message call is accepted and a "call" message or a "response" message is transferred from the network, the process exits from step 202 or 203 to the process of steps 301 → 302 → 404 and immediately “disconnects” the network. A message is sent to execute the disconnection recovery sequence, the telephone number at this time is determined as a valid number, and the fact that the investigated telephone number is valid is displayed on the display 3 of the telephone 1.

In the call setting sequence 200, when the call of the transmitted "call setting" message is not accepted and the "disconnection" message is transferred from the network, the process exits from step 204 to step 401 → 402. , Immediately execute the disconnection recovery sequence,
The reason indication of the information element attached to the "disconnect" message from the network is picked up, and according to the content of the cause indication,
A determination is made as to whether the "call setting" message is valid, invalid, or on hold for the telephone number.

(A) Effective number The reason indication picked up in step 402 is as follows: “Reason number 3 No route to the other party” << Reason number 7 Incoming call to the already set channel >><< Reason number 16 Normal disconnection >><< Reason No. 17 Received User Busy << Reason No. 18 No Called User Response >><< Reason No. 19 Called User Calling / No Response >><< Reason No. 20 No Subscriber >><< Reason No. 21 Communication Rejected >><< Reason No. 27 Remote Terminal Failure << Reason No. 49 QOS Unavailable >><< Reason No. 50 Requested Facility Not Contracted >><< Reason No. 57 Transmission Capability Not Allowed >><< Reason No. 58 Currently Unavailable Transmission Capability >><< Reason No. 63 Other Service or Option Unavailable Class 《Reason number 65 Unprovided transmission capability designation》 《Reason number 66 Unprovided channel type designation》 《Reason number 69 not provided Facility request >>"Reason number 70 Restricted digital information transmission capability"<< Reason number 79 Other service or optional unprovided class >><< Reason number 88 Terminal attribute mismatch >> Is displayed on the display 3 (step 403 → 404).

(B) Number change If the reason indication picked up in step 402 is "Reason number 22 Other party number change", a new telephone number entered in the diagnostic information field of the reason indication is picked up. The fact that the new telephone number has been changed is displayed on the display 3 (step 403).
→ 405 → 406). How the clerk decides in this case will depend on the content of the individual margin transaction.

(C) Invalid number If the reason indication picked up in step 402 is any one of << reason number 1 missing number >><< reason number 2 no route to designated relay network >><< reason number 6 channel not available >> Identifies the telephone number in the "call setting" message as an invalid number, and displays on the display 3 that the telephone number is invalid (steps 403 → 405 → 407 → 40).
8). In addition, instead of making a conclusion only by one investigation processing, a second investigation processing is performed after an appropriate time, and if it is determined that the result is invalid, an invalid display may be displayed. Good.

(D) Judgment suspension If the reason indication picked up in step 402 is not any of the reason numbers checked in steps 403, 405, and 407, the telephone number in the "call setting" message is regarded as a valid number. The determination is suspended without determining the invalid number, and both the OK display and the NG display are blinked on the display 3 (steps 403 → 405 → 407 → 40).
9).

=== Actual Operational Status === The subscribers of the credit transaction to which the present invention is directed are ordinary people, many of which are subscribers of the general analog telephone network. The number of ISDN (INS64) subscribers is increasing. Under the present situation, when the above-described telephone number checking process is executed, the following response is made between the digital telephone 1 and the ISDN exchange (network).

(A) In the case where the called telephone number is a valid subscriber number of the analog telephone network, this number is the largest. Since the digital telephone 1 is calling by specifying the unrestricted digital information as the transmission capability in the "call setting" message, the network returns a "disconnection" message with "reason number 3 no route to the other party". Come. In response, the display 3 displays "invalid". What should be noted here is that no call is made to the telephone to be investigated in making this investigation and determination. In other words, from the point of view of the person being investigated, there is no hassle of forcing an unnecessary call to be answered.

(B) If the called telephone number has been changed, regardless of whether it is an analog telephone network or an ISDN subscriber number, the network will send << reason number 22 change subscriber number >>. It returns the attached "Disconnect" message. In response to this, the display 3 displays "changed to a new telephone number".

(C) When the outgoing telephone number is an unused number It is important to catch this situation as soon as possible in the credit audit of the transaction. In this case, irrespective of the subscriber number of the analog telephone network or ISDN, the network returns a "disconnection" message with << reason number 1 missing number >>. In response, the display unit 3 displays "Phone number is invalid".

(D) When the Called Phone Number is a Valid Subscriber Number of ISDN In this case, the exchange changes according to the situation of the partner equipment.
When a call is made at the receiving terminal (partner) by the "calling" message from the telephone 1, and the partner answers the call, the process proceeds to step 301 in the flowchart of FIG. A recovery sequence is started, and the telephone number is determined to be valid. Also, even if a "disconnect" message is returned from the network,
As described above, the reason numbers are # 7, # 16, # 1
7, # 18, # 19, # 20, # 21, # 27, # 4
9, # 50, # 57, # 58, # 63, # 65, # 6
6, # 70, or # 79, the telephone number is determined to be valid. If the reason number is # 2 or # 6, the telephone number is determined to be invalid. Suspends the decision.

It should be noted that it may be considered that the explanation in the embodiment may not be the best one for determining which reason number is to be assigned to "valid", "invalid" or "pending". ISD
Investigate the usage status of the N subscribers and the response of the network to it in more detail, and it is more appropriate to classify a certain reason indication as "valid" instead of "pending" or to classify it as "invalid" May be appropriate. The present invention does not prevent such elastic operation.

[0032]

According to the digital telephone of the present invention,
The clerk inputs the phone number and issues a search order, so that the phone can automatically and quickly process the number search without disturbing the other party. You can know that.
For example, by installing the digital telephone of the present invention at the customer's reception desk and making effective use of the number search function, fraudulent declaring a random telephone number that is not being used at the site where the contract procedure is in progress is prevented. It can be found immediately, and it can effectively conduct an initial credit audit of customers, which is effective in preventing bad debts from occurring. Also, the fact that the phone number has been changed, without any burden on customers,
This is very practical because it is displayed automatically.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a digital telephone according to an embodiment of the present invention;

FIG. 2 is an external view of the digital telephone.

FIG. 3 is a flowchart showing a procedure for checking the number of the digital telephone;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Digital telephone 2 Operation input part 2a Investigation key 3 Display 4 Handset 5 Line interface 6 Protocol processing part 7 Communication circuit 8 Central processing part

Claims (2)

[Claims] 1. A digital telephone connected to an ISDN basic interface, comprising: a handset, an operation input unit including a number key for inputting a telephone number and the like and a mode selection key for designating an operation mode. , A display for displaying the telephone number entered at the operation input section and the result of the number survey described below, and a call protocol for executing a protocol for talking with the other party of the telephone number entered in the call mode. A digital telephone comprising processing means and number checking means for executing the following processes (1), (2) and (3) for a telephone number input in a search mode. (1) In accordance with the circuit switching call control procedure specified in ITU-T Recommendation Q.931, "call setup" in which the telephone number to be investigated is the called number and unrestricted digital information or restricted digital information is specified as the transmission capability Send the message to the network. (2) When the call of the transmitted "call setting" message is accepted and the "call" message or the "response" message is transferred from the network, the "disconnection" message is immediately transmitted to the network to execute the disconnection recovery sequence. Upon execution, the fact that the telephone number was valid is displayed on the display. (3) If the disconnection message is transferred from the network without receiving the call of the transmitted "call setting" message, the disconnection recovery sequence is immediately executed, and the "disconnection" message from the network is added. The contents of the reason display of the information element being analyzed are analyzed to determine whether the telephone number is valid or invalid, and the result of the determination is displayed on the display. When the content of the reason display is "missing number", the fact that the telephone number is invalid is displayed on the display. 2. The method according to claim 1, wherein the number checking means analyzes a reason display of an information element attached to the “disconnection” message transferred from the network, and determines that the content of the reason display is “partner subscription”. In the case of `` user number change '', obtain a new telephone number entered in the diagnostic information field of the reason display and display on the display unit that the corresponding number has been changed along with the new telephone number A digital telephone characterized by the following.