In 1955 GEC decided to renumber all their telephones with the prefix TEL.  The MT 41xx, MT 42xx, MT 43xx and MT 44xx series, which was the original part number range for the this series of phones were part of this renumbering.  Please see the table below for more information.

The GEC Extension Telephone System

The G.E.C. Extension Telephone System permits two or three telephones to be associated on one exchange line, for exchange calls to be made or received at any of them and for intercommunication between them.  One instrument - the switching telephone - is fitted with a four position switch; this allows calls to be extended to the other two, which are called extension telephones. Inter-telephone calls are initiated by operation of push buttons and signalled by means of buzzers,  or, alternatively, can be signalled over pole routes by magneto calling.


For the Telephone Administration
The system replaces the old switching bell set, with its multiplicity of parts, by a compact telephone which is simple to use and pleasingly modern in appearance.  Subscribers will welcome the special facilities offered, with resultant increased revenue to the Administration.  The switching telephone is already widely used by the British Post Office, for whom it was designed by the G.E.C., and is standardised as Telephone No. 248 with Bell Set No. 39.

For the Business House
In business or commercial premises, the system can be connected to any type of exchange line and proves a valuable aid to speed and efficiency. Incoming calls can be filtered through the switching telephone, under the control of a secretary, and be instantly switched, as required, to the executive or his assistant.

Simplicity of Installation
The system is simple to install, requiring only the following apparatus and connexions:-

  1. At the switching position - a switching telephone, with an external bell to signal exchange calls and a 3-volt battery for inter-telephone calls.
  2. Between positions - a 4-wire interconnecting line of any reasonable length. Line, bell and battery connexions are made at moulded plastic terminal blocks, which have robust construction and pleasing appearance.
Switching Telephone - MT 4150

Switching telephone
The switching telephone comprises a handset instrument, with black plastic case and un-tarnishable metal parts, surmounting a detachable base which accommodates the four-position switch, pushbuttons, buzzer, engaged signal and suppression unit.

Four-position Switch - The switch is robust in construction and foolproof in operation. Its four positions are arranged in logical sequence for easy operation and are shown in the picture to the right.

Pushbuttons - The pushbuttons (G.E.C. Patent No. 493,649) are self-locating in their holes in the case, and are readily detachable from the springsets.  One or two pushes may protrude through the case, as required, without alteration of wiring. When only one push is in use, the second can be neatly stored within the base in a clip formed by one of the contact springs.

Engaged Signal - A transparent aperture in the base displays an engaged signal whenever a conversation is in progress from exchange to extension.

Extension Telephone - MT 4301

Extension telephone
The extension instrument utilises the same case mouldings as the switching telephone, and is entirely self-contained, with the bell (exchange calls) and buzzer (local calls) mounted within the base.  A pushbutton is provided for calling the switching telephone.

Special Features
The telephones incorporate the following features:-

External Bell
For signalling exchange calls at the switching telephone, suitable bells are those of the G.E.C. Cat. No. BE 1100 series (standard) and BE 1200 series (tropical).

Automatic or Manual
The telephones illustrated are for use on automatic systems. Alternative instruments are available for C.B. manual systems, the dial being replaced by a moulded dial dummy. In other respects the C.B. telephones are the same as the automatic instruments and offer the same facilities.

Magneto calling over long Extension lines
Where line wires between switching and extension telephones have to run over a pole route, the convenience of buzzer calling may be sacrificed in favour of magneto calling with the attendant advantage of two line wires instead of four between main and extension. A generator fitted in a separate box, together with a combined bell and capacitor, can then be installed at the main station, and a generator and capacitor at the extension position.

Catalogue Numbers
(Numbers in brackets are the later 1955 catalogue numbers)

MT 4151
MT 4250
MT 4251
MT 4302
MT 4401 MT 4402

Switching telephone - 11lbs / 5kgs
Extension Telephone - 7lbs / 3.2kgs

Taken from the GEC Catalogue MTL 2

G.E.C. Exchange Line Switching Telephone System

The G.E.C. Exchange Line Switching Telephone System permits selected P.A.X. extensions to be connected to the public exchange, thus providing P.A.B.X. working for P.A.X. users at low cost. The system provides for any one of up to 4 exchange lines to be connected to any one of up to 10 selected extensions, which can make or receive public exchange calls The selected extensions are equipped with switching telephones. These are similar in appearance and performance to the "G.E.C. 706 " Telephone described in G.E.C. Catalogue Leaflet No. STL.16, but are mounted on a plinth in which there are six pushbuttons.

The four exchange lines are connected to four of the pushbuttons, each of which can be illuminated individually, either continuously or intermittently, to indicate the state of the exchange lines when the handset is lifted. The remaining two pushbuttons are for HOLD and RELEASE facilities.

Exchange line calls can be transferred from one switching telephone to another when required.

Incoming calls from the public exchange are signalled on bells external to the telephone instruments, the number of bells depending on the location of the switching telephones, e.g., where several switching telephones are located near to one another, only one bell would be required to serve all the telephones.

Both audible and visible warnings are given to an extension engaged on a call to the public exchange if a second extension becomes connected to the same exchange line.

Arrangements can be made for a particular extension to answer all the incoming exchange calls at night.


The operation of the G.E.C. Exchange Line Switching Telephone System is by simple depression of push buttons in the plinth. All push buttons are clearly marked and are used in the following manner:

Incoming Public Exchange Line Calls
An incoming exchange line call can be signalled in one of three ways:-

  1. A particular extension may be detailed to answer all exchange line calls and transfer them to the wanted extension. In this case the incoming call would be arranged to ring only the external bell associated with the answering extension.

  2. An incoming call may ring bells at all the switching extensions. The call would then be answered by the first extension to lift the handset and transferred, if necessary to the wanted extension.

  3. A calling signal switch may be fitted which automatically distributes an incoming calling signal to the extensions in a fixed sequence.

When an extension lifts the handset to answer an incoming exchange line call, the lamp in the push button associated with the calling line flashes in synchronism with the exchange ringing current. To answer the call, the extension simply depresses the push button. The lamp then glows continuously and the extension is connected direct to the public exchange line for speech.

If an extension that is participating on an exchange line call wishes to speak to another PAX extension during the call he presses the HOLD button. This causes a holding loop to be connected across the exchange line and connects the telephone to the PAX. The extension can then dial the number of the wanted PAX extension in the normal way and make an enquiry secret from the exchange line. At the end of the the enquiry call, the extension resumes his conversation on the exchange line by again depressing the appropriate exchange line push button. Any enquiry call can be made to any extension on the PAX.

An extension participating in an exchange line call can transfer the exchange line to any of the extensions having a switching telephone, if required. The same procedure is adopted as for an enquiry call except that when the called extension answers he is informed that he is wanted on a particular exchange line. The second extension then depresses the appropriate exchange line pushbutton and the first extension replaces the handset and presses the release key.

When a switching telephone user wishes to make a public exchange call, he lifts the handset and depresses a pushbutton that is not illuminated. Pushbuttons that are illuminated indicate that the associated exchange lines are engaged. As soon as the pushbutton is depressed the telephone is connected to the public exchange and the pushbutton is illuminated.

When the non-locking release pushbutton is depressed, all locking pushbuttons previously engaged restore to normal. The release pushbutton should be pressed at the end of all main exchange calls. This reverts the switching telephone to normal P.A.X. operation, releases the main exchange line and restores the associated pushbutton to the normal position. Failure to release an exchange-line pushbutton would leave the exchange line connected to the particular telephone.

Should a switching telephone attempt to gain connexion to an exchange line that is already in use, or should two telephones answer an incoming call simultaneously, an interruption tone is heard on the telephone and the pushbuttons associated with the affected exchange line flash.

Arrangement can be readily made for one of the switching telephones to answer incoming exchange line calls at night when most of the telephones are unattended.

To make extension-to-extension calls over the P.A.X. it is not necessary to operate any keys on the plinth. Connexion is obtained in the normal manner by lifting the handset and dialling the required number.

A further pushbutton can be fitted in the telephone instrument to give the normal executive right-of-way facility provided as an optional facility on all G.E.C. private exchanges. This is particularly useful for switching extensions, who regularly have to transfer exchange calls, to enable them to contact extensions who may be engaged on internal calls.

Apart from the switching telephones for those extensions who require to make public exchange calls, the only equipment required additional to the standard P.A.X. is a relay set for each exchange line.

Exchange Line Switching Telephones are available in black, ivory, and dual grey.


Number of Exchange lines required Description of Equipment necessary Catalogue Number
26 volt 50 volt
1 Exchange Line Switching System (Automatic  Equipment) ELS261 ELS501
2 Exchange Line Switching System (Automatic  Equipment) ELS262 ELS502
3 Exchange Line Switching System (Automatic  Equipment) ELS263 ELS503
4 Exchange Line Switching System (Automatic  Equipment) ELS264 ELS504
1 Exchange Line Switching System with Calling Signal Switch (Automatic Equipment) ELS271 ELS511
2 Exchange Line Switching System with Calling Signal Switch (Automatic Equipment) ELS272 ELS512
3 Exchange Line Switching System with Calling Signal Switch (Automatic Equipment) ELS273 ELS513
4 Exchange Line Switching System with Calling Signal Switch (Automatic Equipment) ELS274 ELS514


  Catalogue Number
  26 volt 50 Volt
Exchange Line Switching Telephone complete with plinth 54460/ATT-1 54460/ATT
Exchange Line Switching Telephone complete with plinth and pushbutton
for Executive Right-of-Way facility 
54470/ATT-1 54470/ATT

Please state quantity and colour of the telephone required with the catalogue number.

Taken from the GEC Catalogue Leaflet PXL7

GEC 746
105 and 107 Switching Telephone Systems




The GEC746 105 and 107 switching-telephone systems permit a 'main' (switching) telephone and one or two extension telephones to be connected to one exchange line. The basic telephones are similar in appearance and performance to the GEC746 table telephone. However, one telephone in the system is mounted on a plinth in which there are six pushbuttons to allow calls to be extended to the extension telephones; these latter instruments have a single pushbutton.

The system is particularly suitable, for example, for a doctor who needs to receive emergency calls 24 hours a day and where it is desirable to have only one telephone number. The 'main' telephone would be at his surgery and the extension telephone at his home. On leaving his surgery, he would operate the EXTN TO EXCH button and all calls are automatically routed direct to an extension telephone in his home. This is typical of many similar applications (such as the small business with a separate workshop) where the telephones need to be separated by a considerable distance.

Another application is where one or two executives wish their secretary (in an adjacent office) to accept all incoming calls and only pass on those which require their personal attention. If the executive wishes to make an outgoing call, he can either ask the secretary to dial the number (and transfer the call to him when the called party answers) or dial the number himself having asked the secretary to switch the line to his telephone.

The system also provides an intercom system between the telephones, but under all conditions of usage, an incoming call can always be given attention.


'105 and 107' switching telephone systems use the GEC746 table telephone with the necessary switches, lamps and buzzers added to give the features described in this leaflet; there are four basic versions:
105 internal: 'main' + 2 extension telephones All telephones are in the same building and
107 internal: 'main' + 1 extension telephone interconnexion is by way of a 6-core cable,
105 external: 'main' + 2 extension telephones interconnexion Extensions telephones not in the same building as each other and/or the 'main' telephone;
107 external: 'main' + 1 extension telephone is by way of a 2-core cable.

Case and handset moulded in ABS high-impact plastic, dual grey (the handset, cords, dial surround, and switching plinth are the darker tone). Black and ivory telephones also available. Extension telephones can also be supplied in blue, red, yellow and dual-green.

PVC-covered cadmium-tinsel-conductor cords. Held by grommets to relieve conductors of stresses.
Line cord, straight: 1830 mm (72 in).
Handset cord, coiled: 250 mm (10 in) unextended; 1700 mm (68 in) extended.

Cradle switch
Sealed microswitches.

Characters printed black on antique-silver background beneath clear plastic finger plate. Trigger type and self-compensating action. Projects through self-centring ring on case. No appreciable variations in pulse ratio after 4'/2 million impulses.
Speed: 10 IPS (20 IPS version available). Inter-digit pause: 240 ms minimum.
Break-to-make ratio. 2 to 1 (other ratios can be supplied).

Receiver inset: rocking armature, replaceable capsule; sensitivity, + 56 dB relative to 1 Pa/root W available power.
Transmitter inset: carbon granule, replaceable capsule; sensitivity -10 dB relative to 1 V/Pa at 2 Pa input.

An automatic volume regulator is fitted to limit speech-signal amplitudes on lines shorter than 400 ohms; introduces virtually no attenuation on longer lines.

Exchange: double-coil ringer, DC resistance 1000 ohms; accepts ringing currents between 16 and 30 Hz; bells, double gong, harmonizing tones.

Telephone separation: the line-loop resistance should not exceed 1000 ohms between the exchange and the most remote extension. On 'external' systems the maximum mutual capacitance due to the main-to-extension lines should not exceed 0.45 uf - equivalent to a total extension-line length of the order of 9 km (6 miles) when using some of the common 0.5 or 0.6 mm (6.5 or 10 lb) cables.

Intercommunication : DC single-coil buzzer, moving armature type; DC resistance 55 ohms. On 'external' systems, signalling is via a 20 Hz AC ringing convertor and the normal telephone bell.

Suitable for use in tropical climates. Protected against entry of insects.

Dimensions and Weight

  'main' telephone extension telephone
Height 152 mm (6 in) 119 mm (4.75 in)
Width (over handset) 250 mm (10 in) 250 mm (10 in)
Width (over case) 140 mm (5.5 in) 140 mm (5.5 in)
Depth 248 mm (9.75 in) 216 mm (8.5 in)
Weight (complete telephone) 2.7 kg (6 lb) 1.7 kg (3.5 lb)
Weight (handset) 230 gm (8 oz) 230 gm (8 oz)

Catalogue Numbers
A complete system requires:
One 'main' (switching) telephone type TST4001.
One or two extension telephones, with a single pushbutton type TST1710.
One DC supply: either 12 V DC battery or GEC power unit type D97206T (200-250 V AC input) or type D97491 T (100-125 V and 200-250 V AC input).
One AC ringing convertor (required for 'external' systems), type D97130T.

Optional facilities
Pushbutton telephones: the telephone can be supplied with a pushbutton keyset. Two versions are available: one provides loop-disconnect (decadic) signalling, 10 or 20 IPS, and the other provides DC-leg signalling for suitable PABX systems.
Loudspeaking telephones: GEC loudspeaking telephones LST4D can be used for the extension telephones. Direct calling between extensions: on 'internal' systems, a second pushbutton on extension telephones can be provided for inter-extension direct calling.

Taken from the GEC Catalogue Leaflet DLT32B 3ME879 (1971)

See also TEL/9S

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Last revised: 25 October, 2008