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TEL/9E - TEL/14E & TEL/9S - TEL/10S

GEC Extension Telephone System

The G.E.C. Extension Telephone System permits two or three telephones to be associated on one exchange number.  Exchange calls can be made or received by each telephone as required, and any two telephones can converse with each other independent of the exchange.

The basic telephones are similar in appearance and performance to the 'New Gecophone' described in G.E.C. Catalogue Leaflet STL 16.

However, one instrument, the main or switching telephone, is mounted on a plinth in which there are six pushbuttons to allow calls to be extended to the extension telephones.  These extension telephones have either a single pushbutton to call the main or two, three or four pushbuttons to provide other facilities as described below.  In business or commercial premises the system allows incoming calls to be filtered through the switching telephone under the control of a secretary and be switched to the executive or an assistant as required.  Executives however, can make calls direct to the main exchange without the intervention of the secretary at the main instrument if the extension to exchange press button is left depressed on the switching telephone.   The system also provides a private intercommunication system between the executive, the assistant and the secretary, but under all conditions of usage a call from the main exchange can always be given attention.



  • Calls from the exchange can be answered at any telephone.  Normally they are answered at the main telephone and transferred to either of the other telephones when required.
  • Outgoing calls can be made from any telephone, as required.
  • Calls between the main telephone and the exchange are secret from the extension; calls between an extension and the exchange can he secret or non-secret from the main.
  • Intercommunication calls between the main and extension are secret from the exchange and can be made while an exchange call is held.
  • It can be arranged that when the exchange line is left connected to an extension, an incoming call ring both the main and extension bells.
  • Both the extensions can be either internal or external.
  • Signalling between the main and an extension is either by D.C. or by AC. from a 20c/s transistor ringing converter for internal extensions and by A.C. for external extensions.
  • As with the 'New Gecophone/GEC706 telephone,' satisfactory operation is obtained (from a 50-volt 200/200 ohm exchange) over lines having a loop resistance of up to 1120 ohms, either between the exchange and the main telephone where the extensions lines are short and internal, or between the longest extension and the exchange where external extensions are used. the system can also be connected to other types of telephone exchange.
  • The system can be used on either automatic or C.B, manual lines, on either exchange or shared-service lines.  When the system is used on a shared-service line, an additional pushbutton is required in each instrument.
  • No modification is required to apparatus at the exchange.
  • These telephones were designed in conjunction with the British Post Office and are the only telephones now accepted by them giving full Plan 5 and Plan 7 facilities.


TEL/9S/ATS - Switching Telephone

TEL/9S/ATS - Switching Telephone
Internal view of the base unit.  The wires hanging are for connection to the telephone.

The operation of the system is by simple depression of pushbuttons.  These are clearly marked and perform the following functions:-

Speak to Exchange
This connects the main (switching) telephone to the exchange and allows calls to be originated or received in the normal manner.  All conversation is secret from the extension.  Should this key be left in the operated position, incoming exchange calls ring the bell at the main only.

Speak to Extension (Exchange Held)
This connects a resistor across the exchange line and allows the main to talk to the extension while an exchange call is held this conversation is secret from the exchange.  A red panel behind this key is illuminated as long as the main handset is lifted while the key is depressed.

Extension to Exchange
Should the main now wish to transfer the exchange call to the extension, he does so by simply pressing this pushbutton.  The conversation is secret or non-secret from the main as required.  A panel behind the key in the switching telephone remains illuminated while the extension handset is off the rest.  If the key is left operated, the extension can originate exchange calls and incoming calls ring the extension bell.  Incoming calls can also ring the main bell simultaneously, when required.  Internal extensions can recall the main while connected to the exchange.

Main to Extension
By depressing the appropriate key, the required extension is called.  Both simply lift the handset to speak.  Similarly, the extension can call the main by operating a pushbutton.

By depressing this key, which is non-locking, the locking keys that have been engaged are restored to normal.  When all the keys are normal, incoming exchange calls ring the bell at the main only.

A single pushbutton on the telephone allows the extension to call the main.  Two, three, or four pushbuttons are fitted to an extension telephone to provide other facilities such as direct calling between extensions, executive right-of-way and recall or shared service.

The plinth of the switching telephone is connected by a cord to a terminal block at which all the necessary external connexions are made to the exchange, extensions, power supply, etc.

A multi-way cable is required to connect the main telephone to each extension.  The number of wires in the cable depends upon the facilities required.  The diagram in Fig. 1 shows the requirements for a typical internal system with two extensions having direct calling between extensions.  Detailed information relating to circuits and external connexions, cabling, etc., is supplied with each order.

A single pushbutton is fitted in external extensions and is used for calling the main.  Direct calling between two extensions is not provided, but an extension can call the main, which can, in turn, call the second extension.  When the called instrument handset is lifted conversation can proceed between extensions.

The plinth of the switching telephone is connected to a terminal block in a similar way to the internal system.  A two wire cable is required to connect the main to each extension.  A separate ringing converter unit is required for external working.  This is connected to the terminal block to provide the ringing current to call over the two wires to an external extension.  Ringers on the main and extensions must be connected one side to earth.  Detailed information relating to circuits, external connexions, cabling, etc. is supplied with each order.

A.C. Ringing Converter (cover removed)

The switching telephones, and extension telephones having more than one pushbutton are currently produced in black, ivory and dual grey.  Extension instruments having a single pushbutton can be supplied in black, blue, ivory, red, topaz yellow, dual grey and dual green.

Power Supply
A 12 volt d.c. supply provides the main-to-extension talking current, the current for the switching telephone indication lamps and the ringing converter supply, when required.  A small mains unit can be supplied for this purpose or a battery of dry cells can be used.

Maximum Line Length
The maximum line resistance for the system is the same as for the GEC706 telephone i.e. the maximum resistance from the exchange to an extension is 1000 ohms.  The sum of the main-to-extension lines should not exceed six miles when cable of approximately 0.075 uF/mile is used.

The switching telephone and the extension telephones are designed and fully tested to withstand the rigours of service in all climatic conditions in countries throughout the world.

As the basic circuit and many of the components of these telephones are the same as those used in GEC706 telephones.  An  administration using both types can have a common set of spares for many items in all instruments.

Catalogue Numbers

Standard  Tropical
Switching Telephone
Extension telephone (one push button)
Extension telephone (with second pushbutton for direct calling between extensions, or other special facility) TEL/13E/ATS

Please state colour of telephone required after catalogue number and whether the extension telephones are for internal or external systems.  Where additional pushbuttons are required to provide other facilities, details of these should be given with orders or enquiries.

Taken from the GEC Catalogue Leaflets STL 17, 17A and 17B

Click here to see the article on Plansets

Additional Pictures

Hong Kong Variant of Tel/10E/ATT
To diagram KS13254

Front view showing Hong Kong Telephone Companies name.
Dial Label is not correct
Internal view showing Buzzer fitted on right hand bell fixing



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Last revised: January 30, 2021