GB2157130A - Telephone apparatus - Google Patents

Abstract

The invention relates to a key telephone system comprising a control circuit for interfacing with a plurality of extension lines (1 to 5) and exchange lines (6 and 7), each extension comprising a standard telephone (13). A keyboard (17) is connected to one extension telephone (13) and is interfaced with the control circuit. The keyboard (17) comprises a plurality of push-button switches (S1 to S7), namely, switches (S1 to S4) for connecting the keyboard (17) to any of the remaining extensions and switches (S6 and S7) for connecting the extension to any of the exchange lines (6 and 7). The control circuit is controlled by a microprocessor (12), which also includes means for storing a plurality of numbers which may be dialed on a signal from an extension line, and also for storing the last dialled number from any extension for subsequent redialling. <IMAGE>

Description

SPECIFICATION Telephone apparatus The present invention relates to a telephone key system, and in particular, to a control circuit for controlling the key system.

Telephone key systems in general comprise a plurality of extension units, each with a rotary dial or press button dial, together with a plurality of keys for connecting the extension to one or more outside lines, or to another extension line. All known key telephone systems suffer from the major disadvantage, in that each extension must have its own built-in control apparatus to control the extension and the flow of calls to and from the extension. This leads to considerably expensive apparatus. A further disadvantage is that a large number of connections have to be made between each extension, thereby necessitating relatively bulky and expensive cabling.

The present invention is directed towards providing a telephone key system which may be used with standard telephones and an associated keyboard, and it is also directed towards providing a control circuit for the system.

According to the invention there is provided a control circuit for a telephone key system, the control circuit comprising a circuit to receive at least one exchange line and at least two extension lines, a switch matrix to connect the exchange line to an extension line, or two extension lines together, control means provided by a microprocessor for controlling the operation of the switch matrix and the circuit, and interface means to interface the control circuit with one remote telephone extension.

In one embodiment of the invention, the switch matrix comprises a plurality of relays.

In another embodiment of the invention, means to detect current flowing in each extension, and each exchange line, is provided.

In a further embodiment of the invention, the means to detect the current is provided by an optocoupler in each line, and means to communicate the presence or absence of current flowing to the microprocessor, is provided.

In a further embodiment of the invention, means to detect a break in an exchange line is provided, and means to deliver a signal to the microprocessor in the event of a break in in the exchange line, is provided.

Advantageously, the means to detect a break in an exchange line is provided by an opamp.

In one embodiment of the invention, means to store a plurality of telephone numbers is provided in the microprocessor, and dial means to dial the stored numbers provided in the microprocessor, the numbers being recalled and the dial means being activated by a code signal delivered from an extension line by dialling at least one digit on the extension telephone. Further, means to store the last number dialled by an extension is provided in the control means, and means to redial the last number stored, is activated by a code signal delivered from an extension line by dialling at least one digit on the telephone.

In another embodiment of the invention, the microprocessor comprises means to selectively direct calls from an unattended extension to another extension, and means to store the number of the extension to which the call is to be directed.

Additionally, the invention provides a telephone system comprising the control circuit and a standard telephone for each extension line, and a keyboard associated with one of the standard telephones, the keyboard being connected to the control circuit by interface means.

Advantageously, the keyboard comprises a plurality of push-button switches, one for each telephone extension line for directly selecting an extension, and one for each exchange line for directly selecting an exchange line.

In another embodiment of the invention, a call control push-button switch is provided on the keyboard.

In a further embodiment of the invention, a plurality of light means are provided corresponding to each push-button switch, to indicate the status of each exchange line and each extension line.

Figure 1 is a circuit diagram of the control circuit according to the invention; Figure 2 is a perspective view of a telephone and associated keyboard for use in the system according to the invention.

Referring to the drawings, and initially to Fig. 1, there is provided a control circuit for controlling a telephone key system, having five extension lines and two exchange lines. The extension lines are indicated generally by the reference numerals 1 to 5, and the exchange lines are indicated by the reference numerals 6 and 7. The extension lines 1 to 5 are connected to the exchange lines 6 and 7 through a switch matrix 9, provided by relays RL15 to RL24. A control means, in this case provided by a microprocessor indicated generally by the reference numeral 12 controls the operation of the control circuit and the relays in the switch matrix 9.

Each extension line 1 to 5 is connected to a standard telephone 13 with a push-button dial 15. One of the extension telephones, normally, the secretaryxoperator's extension is connected to the control circuit through a keyboard 17. The keyboard 17 is described in detail below with reference to Fig. 2, and as can be seen in Fig. 2, forms a plinth 14 onto which the telephone 13 is mounted. The microprocessor 12 receives instructions from the keyboard 17 and from each telephone dial 15, and the microprocessor also receives instructions from the exchange line 6 and 7.

A tone generating circuit 16 is connected to each extension line 1 to 5 through capacitors C10 to C14, and coupling transformers TR1 to TR5. A ring generator circuit 18 is connected onto each extension line 1 to 5 through relays RL5 to RL9.

A current detection means, in this case optocouplers 103 to IC7, is provided in each extension line 1 to 5 respectively, for supervision of the current loop. The optocouplers IC3 to IC7 are supervised by the microprocessor IC16 through an octal buffer IC11 through lines 24. A t5 volt supply is connected to ground through a resistor group 23, the lines 24, diodes D2, D4, D6, D8 and D10, and the optocouplers IC3 to IC7. Each resistor in the resistor group 23 corresponds with an optocoupler IC3 to IC7. For simplicity, the integrated circuit IC3 and its related circuitry will now be described.The optocouplers IC4 to IC7 and their related circuitry, are similar.

Once current flows in extension line 1-, in other words once the loop is closed by an off hook condition on extension telephone 1, the light emitting diode in the optocoupler IC3 causes the transistor in the optocoupler IC3 to conduct the 5 volt supply from the resistor group 23 to ground. Thus, the appropriate line 24 is at ground potential. The octal buffer on sensing this, delivers a signal to the microprocessor IC16 that current is flowing in extension line 1.

An exchange line break detector to detect a break in the exchange line, is provided in extension line 1 by an opamp IC8. The opamp IC8 prevents loop current interruption originating at the parent exchange from simulating dialled breaks. Should a break occur in the exchange line, the opamp holds the potential on the appropriate line 24 to ground, to avoid the microprocessor receiving a spurious signal. This is important where the microprocessor is monitoring a number being dialled for subsequent redialling. The opamp IC8 is connected to the positive line of extension 1 through a potential divider provided by resistors R80 and R75. A current limiting resistor R71 connects the other side of the opamp to the ground line of extension 1.The output of the opamp is connected through a current limiting resistor R60 and a resistor R50 to the diode D2. Zener diode 10 connects the resistor R60 to ground to prevent the output voltage from R60 exceeding +5 volts. In normal operation when current is flowing in the exchange line, the opamp IC8 delivers an output voltage which is conducted to ground through IC3. However, on a break occurring in the exchange line, the inputs to the opamp 108 fall to zero volts, and thus the output from the opamp IC8 also falls to ground. This therefore holds the line 24 at ground while the exchange break occurs. This thus permits the microprocessor IC16 to continue to receive a dialled number being dialled from an extension.The opamps IC9 to IC10 operate in similar fashion on the extension lines 2 to 5. Zener diodes ZD5 to ZD9 are provided to protect the optocouplers IC3 and IC7.

In the absence of the opamps IC8 to iC10, the microprocessor could store incorrectly dialled numbers with consequent errors in retransmission, when the repeat number dial facility is used, as will be described below.

Protection circuits 20 are provided on each exchange line 6 and 7. Optocoupler circuits IC1 and IC2 in the exchange lines 6 and 7 detect ringing from the exchange. Relays RL1 to RL4 are connected in the exchange lines, so that in the event of power failure (when the contacts are as shown in Fig. 1), exchange line 6 is fed to extension line 1, and exchange line 7 is fed to extension line 2. The relays RL1 to RL4 also provide facilities for dialling stored numbers in the microprocessor memory, and permit a call to be placed on hold while a second call is being made.

A power supply circuit 19 delivers +5 V DC and +12 V DC power suppliers to the circuit. The 12 V supply powers the relays RL1 to RL9 and the relays RL10 to RL24 in the switch matrix. The tone generator circuit 16 is powered by the 5 V supply which also powers the microprocessor as will be described below.

The microprocessor 12 comprises a central processing unit IC16, in this case an RCA1802ACE, and a ROM 1017 and a RAM IC18 provided by an AM9128. The programme to control the operation of the microprocessor, is stored in the IC17. A plurality of telephone numbers are also stored in the RAM IC18.

The microprocessor 12 is programmed to dial any of these numbers on a signal from an extension line.

Each stored number is identified by dialling two digits on the telephone dial, which corresponds to the desired stored number. On receiving the signal from the telephone extension dial, the microprocessor selects the appropriate number and dials it, once one of the exchange lines 6 or 7 become free. The RAM IC18 also stores each last dialled number from each extension. Thus, in the even that a call is not set up on the first dialling, by delivering a signal from the telephone extension to the microprocessor, the last call to have been dialled is redialled. This signal from the telephone extension is delivered by dialling one or two digits on the extension. The RAM IC18 also stores a plurality of call codes which may be barred to one or all of the extensions.On an extension telephone attempting to set up such a barred call, the microprocessor prevents the call being set up.

Address decoders, namely, integrated circuits IC19, IC22 and IC23, are also provided. Latch drivers provided by integrated circuits IC12, 1013 and IC14 are provided to drive the relays. The latch driver IC12 drives the relays RL1 to RL4 and RL5, RL10, RL15 and RL20. The latch driver IC13 drives the relays RL6 and RL7, RL11 and RL12, RL16 and RL17, and RL21 and RL22. The latch drive iC14 drives the relays RL8 and RL9, RL13 and 14, RL18 and 19, and RL23 and 24. Each of the latch drivers IC12 to IC14, controls the coupling transformers TR1 to TR5. Interface means to interface the control circuit with the extension keyboard 17 is provided by integrated circuits IC20 and IC21.

The keyboard 17 in this embodiment of the invention comprises push button switches S1 to S4 to per mit the extension to be connected to any one of the remaining four extension lines. Push button switches S6 and S7 are provided for connecting the extension to either outside line 6 or 7. A push button switch S5 is also provided in the keyboard 17 to control the calls. Six light emitting diodes LED 1 to LED 6, are provided in the switches S1 to S4 and S6 and S7. LED 1 to 4 are provided to show the status of the remaining four extension lines, while LED 5 and 6 show the status of the exchange lines 5 and 6.

The operation of the telephone system will now be described.

The following table illustrates the operation of the telephone systems, and numbers to be dialled to select the various facilities.

Table 1 - Numbering plan

0 - Operator/Extension 1 60 -Call Barring 1 Hold/Transfer 61 - Call Parking 2 - Ext 2 62 - Call Diversion 3 - Ext 3 63 - Ring when Free 4 - Ext 4 64 - Enable Central Bell/ Night Switch 5 - Ext 5 65 - Disable Central Bell 6 - Facilities Control ---- 66 - Conference 7 - Repeat Last No. 67 - Call Forwarding Don't Answer 8 - Abbreviated Dialling 68 - Spare 9 - Exchange Line access 69 - Spare Universal Answer Each user may select either of the external lines.

Each extension can select facilities such as exchange line access, hold and transfer by dialling the relevant digit described in the numbering plan of Table 1. In the case of MF telephones (particularly in the Hybrid-PABX application) a telephone with a "timed break" button or recall button should be used. Once the call is in progress this button should be depressed prior to dialling the digits. If such an MF telephone is unavailable, then a brief depression of the cradle springs (hook-flash) will be necessary before invoking the facility. In the following description, the expression "secretarial set" refers to the telephone 13 and keyboard 17 used by the secretary/operator. It is functionally and visually identical to the other key sets. In the Hybrid key system/PABX mode, it describes a single key set used.

The system tones used to indicate call set-up status are as follows: Table 2 - Internal Tones Dial Tone : 425 Hz Continuous Busy Tone : 425 HZ 0.38 Sec On/0.38 Sec Off Ringback Tone : 425 Hz 1.0 Sec On/2.0 Sec Off Alerting Tone : 425 Hz 100 mSec On/2.5 Sec Off Call Diversion Tone 425 Hz 2.50 Sec On/100 mSec Off (Special Dial Tone) Ringing Current : 25 Hz Ringing Cadence - Exchange Line Call : 0.40 Sec On/0.2 Sec Off/0.4 Sec On/2 Sec Off Internal Call 1.0 Sec On/2.0 Sec Off Full Key System Visual indications Configuration Status of Exchange Lines: - Lamp glows for exchange line busy.

- Lamp flashes at 1Hz for exchange line on hold.

- Lamp flashes at external ringing cadence for incoming call.

Status of Extensions: - Lamp glows for extension off hook.

- Lamp flashes at 1Hz for exchange line on hold.

Lamp flashes at internal ringing cadence for internal call.

In the full key system mode, where each extension is equipped with telephone and keyboard, only one extension, the secretarial set, receives ringing current to the telephone bell, although the appropriate LED of the other key boards flash intermittently.

All extensions may answer calls by depressing the appropriate button switch S6 or S7 or by dialling digits as illustrated in the numbering plan (Table 1).

The operation of each extension telephone and keyboard is as follows: General Whenever a user lifts off, system dial tone will be heard in the handset. The user has access to all the system features either via the telephone dial keyboard.

Incoming Call Lamp flashing on associated exchange or extension LED.

Operator lifts handset and presses non-locking button on keyboard.

Outgoing Call User selects exchange line by depressing appropriate button switch S6 or S7. On hearing exchange dial tone normal call set-up may proceed via the telephone push-button dial 15.

Intercom An intercom connection can be set up by depressing the appropriate extension button switch S1 to S4.

Call Waiting Tone Call waiting tone can be induced into a busy extension's circuit by pressing the appropriate extension button switch S1 to S4. The extension LED will flash while call waiting tone is on line.

Call HoldlTransfer Calls are automatically placed on hold when the user depresses an extension button switch. Call transfer is completed when the user hangs up. The user can also place calls on hold without transferring calls by pressing the call control button switch S5, for example, when a personal call is to be held on reception of a second incoming call.

Making an Internal Call 1. Lift Handset - internal dial tone is heard. System busy tone will be heard if all links are busy.

2. Press appropriate extension button switch S1 to S4 - or dial number.

3. LED flashes as extension is rung.

4. LED glows steadily on answer - confer.

Making an External Call 1. Lift Handset - internal dial tone is heard. System busy tone is heard if all links are busy.

2. Press free exchange line button switch S6 or S7 - or dial 9.

3. LED glows to show exchange line is busy.

4. Dial number required.

5. Confer- in private.

Repeat Last Number The control circuit retains in its memory the last number dialled by each extension on an external call.

To redial this number lift the handset and dial '7'. The microprocessor sends out the digits automatically after a delay to ensure that exchange dial tone is received.

Answering Incoming Call 1. Incoming Call rings on Secretarial Set if free. LED also glows with ringing cadence.

2. Each other user gets a visual indication of call arrival by the appropriate LED on his keyboard flashing.

3. Each user may answer by lifting handset and depressing appropriate switch S6 or S7 or alternatively may answer by dialling '9' (universal answer).

4. If night service has not been invoked (see Page 23) or if secretarial extension is busy, then call hunts around all other extensions at intervals of 20 seconds ringing at each telephone extension.

Call HoldlEnquiry 1. Put the call on hold using call control button switch S5 - or dial '1'.

OR 2. Depress the appropriate extension button switch S1 to S4 to ring extension and make enquiry in which case call is automatically put on hold.

3. Revert to outside line by depressing the appropriate line button switch S6 or S7.

Call Transfer 1. When an extension answers, the call is transferred by replacing the handset or by depressing call control button switch S5 again - in which case internal dial tone is heard by the user.

2. If the call is transferred before the extension answers it reverts to the originating telephone exten sion.

Unattended Transfer 1. Depress call control button switch 55 - or dial '1'.

2. Depress relevant extension button switch S1 to S4 - (or dial number) and replace handset.

3. Cailed extension will ring for 20 seconds.

4. If extension is unanswered after 20 seconds, call reverts to originating extension.

Third Party Add on A maximum of 1 exchange line and 2 extensions are permitted on a single conference: 1. Set up outside call in usual manner.

2. Depress call control button switch S5 (or dial '1') to put outside call on 'hold'.

3. Internal Dial Tone is now heard.

4. Select or dial extension required. If it is busy revert to outside line again by depressing call control button switch S5 (or dialling '1'). If free called extension rings. On answer confer in private.

5. Conference between all three parties is then set up by dialling '66' when in contact with the outside line or the extension.

Either extension may subsequently withdrawn by replacing the receiver without affecting communication between the other parties.

Call Diversion Allows extension to divert incoming calls to another host extension. Outside calls are still possible.

1. Lift Off - await dial tone.

2. Dial Diversion Code '62' followed by number of host extension.

3. Diversion is acknowledged by the Special Dial Tone.

4. Special Dial Tone is heard by the extension as a reminder while the diversion facility is in use.

5. The diversion facility will not be permitted if an attempt is made to divert calls to an extension which has already invoked the diversion facility. Acknowledgement tone will not be given in this case.

6. An extension which is already acting as host for diverted calls cannot use the facility to divert calls to a third extension. Acknowledgement Tone not given.

7. The host extension is permitted to call the diverting extension and transfer a call to it.

8. Diversion is cancelled by dialling '62' and then hanging up.

Call Forwarding - No Answer Allows diversion of incoming call to another extension if call is unanswered after 4 rings.

1. Lift Handset - hear internal dial tone.

2. Dial diversion code - '67', followed by number of host extension e.g. 670.

3. Diversion is acknowledged by 'Special Dial Tone'.

4. 'Special Dial Tone' is heard each time handset is picked up.

5. Cancel diversion by dialling '67' and then hanging up.

Night Service This facility is used to ring a particular extension (e.g. extension '4') or a central bell (if fitted instead of a keyboard/telephone). Night service may only be invoked or revoked from Extension '0' namely, the secretarial set.

1. To Invoke: - lift handset, await internal dial tone and dial '64' followed by number of the right service extension e.g. 644.

- dial tone is then heard as acknowledgement.

2. To Revoke: lift handset, await dial tone, dial '65'.

- dial tone is then heard as acknowledgement.

Abbreviated Dialling Each extension may have access to a maximum of 8 abbreviated numbers. The facility is intended for use on exchange calls only.

1. When an abbreviated code in the range 81-88 is dialled, the system will seize an exchange line and after a presending delay of 2 seconds will send the subscriber number corresponding to the abbreviated code.

2. Insertion of abbreviated numbers is carried out from the extension by lifting off the receiver and when internal dial tone is heard, dialling in the insertion code 89. An abbreviation code, for example 1-8, is then inserted followed by the subscriber number (including trunk and international prefixes) which the code will represent. After each abbreviated code is inserted, it is necessary to hang up and lift off again before inserting the next abbreviated number.

Brokerage Calls 1. Set up first outside call. Dial 9 followed by subscriber's number.

2. Dialling 9 puts first outside call on hold - External Dial Tone is now heard on second exchange line.

3. Set up 2nd outside call.

4. Shuttle from one line to the other by dialling 9.

5. If extension hangs up when connected to one line, the call on that line is cleared. The extension rings immediately and on lift off is connected to the remaining outside call.

Call Parking Allows an external call to be put on special hold circuit without ringing another extension.

1. Set up call in usual manner.

2. Park call by dialling control code 61. If parking circuit is busy, then Busy Tone is heard. When call is parked, the extension hears the internal dial tone.

3. Parked call may be retrieved by dialling code 61 from any extension.

4. A call which is parked and remains unretrieved for 30 seconds will revert to the Operator.

5. If parking circuit is busy then the extension may return to the outside call by dialling 6 again.

Call Barring Each extension may be assigned one of the following categories: a. All outgoing calls allowed (category 1).

b. International calls are barred (category 2).

c. All outgoing calls except emergency calls (9) 999 are barred (category 4).

1. At initial switch-on or after a break in mains power, all extensions are assigned to category a.

2. The allocation of categories may be changed from a particular master extension by dialling a five digit code, beginning with the digit 6. The number of each extension is then dialled followed by the category digit for the barring option required on that extension.

3. When all extensions have had category digits allocated, a five-digit code is dialled, which will be the access code for future changes in the category allocation. Access codes must begin with digits 60.

4. After power-up, the access code will be 60000 until changed from the master extension.

Ring When Free When a user encounters a system busy tone when trying to get an external line or an extension, if the user then dials code 63 and replaces the handset the call will be set up when the engaged circuit becomes free and both the calling extension and called extension will be re-rung.

It will be appreciated that while the apparatus has been illustrated as comprising a telephone keyboard in modular form, the keyboard could be provided in an extension telephone namely, as an integral part of the telephone. Further, it will be appreciated that while a push button dial telephone has been provided, a rotary dial or any other conventional telephone could be used.

The particular advantage of the invention and of providing a separate keyboard with the operator extension telephone, is that it permits a conventional telephone to be used as an extension telephone while still retaining the features of the key system.

Further, it will be appreciated that while particular power supply circuits, tone generator and ringing circuits have been illustrated, any other suitable circuits could be used. Similarly, other suitable current and voltage detectors on the lines could be used.

Additionally, it will be appreciated that while a particular arrangement of relays has been illustrated, any other suitable arrangement could be used.

It will also be appreciated that while particular integrated circuits have been described for use in the microprocessor, any other suitable integrated circuits could be used. Additionally, other suitable configurations of integrated circuits could also be used.

It is also envisaged that while the circuit has been described for controlling five extension lines and two exchange lines, any suitable number of extension and exchange lines could be used. Indeed, it is envisaged that the circuit could be used for a case where one exchange line and two extensions were required.

Further, it is envisaged that other suitable interface means besides those described, for interfacing the control circuit with the keyboard, could be provided.

It is also envisaged that while the control circuit has been described as providing a member of additional functions, for example, a means to store calls to be barred, a means to store additional telephone numbers for dialling, a means to store the last dialled number and other additional features, the control circuit could be provided without these features, without departing from the scope of the invention.

Furthermore, it is envisaged that a keyboard could be provided for each extension telephone.

Claims (22)

1. A control circuit for a telephone key system, the control circuit comprising: a circuit to receive at least one exchange line and at least two extension lines, a switch matrix to connect the exchange line to an extension line, or two extension lines together, control means provided by a microprocessor for controlling the operation of the switch matrix and the circuit, and interface means to interface the control circuit with one remote telephone extension.

2. A control circuit as claimed in claim 1, in which the switch matrix comprises a plurality of relays.

3. A control circuit as claimed in claim 1 or 2, in which means to detect current flowing in each extension, and each exchange line, is provided.

4. A control circuit as claimed in claim 3, in which the means to detect the current is provided by an optocoupler in each line, and means to communicate the presence or absence of current flowing to the microprocessor, is provided.

5. A control circuit as claimed in any of claims 1 to 4, in which means to detect a break in an exchange line is provided, and means to deliver a signal to the microprocessor in the event of a break in the exchange line, is provided.

6. A control circuit as claimed in claim 5, in which the means to detect a break in an exchange line is provided by an opamp.

7. A control circuit as claimed in any of claims 1 to 6, in which a dial tone is delivered to an extension line through a transformer.

8. A control circuit as claimed in claim 7, in which the microprocessor enables and disables the transformer.

9. A control circuit as claimed in any preceding claim, in which means to store a plurality of telephone numbers is provided in the microprocessor, and dial means to dial the stored numbers is provided in the microprocessor, the numbers being recalled and the dial means being activated by a code signal delivered from an extension line by dialling at least one digit on the extension telephone.

10. A control circuit as claimed in any preceding claim, in which means to store the last number dialled by an extension is provided in the microprocessor, and means to redial the last number stored is activated by a code signal delivered from an extension line by dialling at least one digit on the extension telephone.

11. A control circuit as claimed in claim 9 or 10, in which the means to store telephone numbers is provided by a random access memory, and the means to dial a stored number is provided in a read only memory.

12. A control circuit as claimed in any preceding claim, in which the microprocessor comprises means to selectively direct calls from an unattended extension to another extension, and means to store the number of the extension to which the call is to be directed.

13. A control circuit as claimed in claim 12, in which the means to direct the call is provided in a read only memory, and the means to store the extension number to which the call is to be directed is stored in a random access memory.

14. A telephone system comprising the control circuit of any of the preceding claims and a standard telephone for each extension line, and a keyboard associated with one of the standard telephones, the keyboard being connected in the control circuit by interface means.

15. A telephone system as claimed in claim 14, in which the keyboard comprises a plurality of pushbutton switches, one for each telephone extension line for directly selecting an extension, and one for each exchange line for directly selecting an exchange line.

16. A telephone system as claimed in claim 14 or 15, in which a call control push-button switch is provided on the keyboard.

17. A telephone system as claimed in claim 15 or 16, in which a plurality of light means are provided corresponding to each push-button switch, to indicate the status of each exchange line and each extension line.

18. A telephone system as claimed in any of claims 14 to 17, in which the keyboard forms a plinth on which to support the telephone.

19. A keyboard for use in the telephone system of any of claims 14 to 18.

20. A control circuit substantially as described herein, with reference to and as illustrated in the accompanying drawings.

21. A telephone system substantially as described herein, with reference to and as illustrated in the accompanying drawings.

22. A keyboard substantially as described herein, with reference to and as illustrated in the accompanying drawings.