600 Type Relay
2000 Type Selector
3000 Type Relay
4000 Type Selector
CBS Systems
Central Battery Systems
Code "C" signalling
Earth Calling Exchange Lines
Earth recall
Extension Telephone Plan arrangements
Local Battery systems
Loop Calling Exchange Lines
Loop Dial signalling
Magneto systems
MF signalling
Party Line
System X
System Y
Timed Break Recall

ITT European Code C. This signalling system was DC based and used on early electronic PABX's.  The telephones had a push button unit that had 12 buttons numbered 1 - 0 and included * and #.  The # button was used for recall/transfer purposes as it sent an earth condition to each wire.  The * button was generally unused.  The diagram below shows the signals used and it must be noted that each telephone would have to have an earth connection.

One of the disadvantages of this system is that the line polarity must be observed.  If a line reversal occurs then conditions on buttons 1 & 2, 4 & 8, 5 & 7 and 6 & 9 will be reversed.

This system was effectively superseded by MF signalling.

Signalling condition with Code "C"

These are normally connected to Telephone Systems and the lines only return dial tone on receipt of a loop on the line, followed by an earth condition.  For more information click here.

This type of recall involves the connection of an earth to one leg of the line.  This is generally used on PABX extensions.  An earth must be connected to 4 on the Line Jack.

This type of exchange line is normally used for residential connections and small keysystems.  The line returns dial tone on receipt of a loop condition on the line.  For more information click here.

This system uses a dial and sends timed disconnection's to line, corresponding with the digit selected i.e. digit 4 equals 4 disconnection's.  Dials are strictly timed to ensure that the pulses are to a set standard.

Signalling System Multi-frequency No. 4 is an AC signalling system.  The codes it sends is made up of a combination of tones, which are produced by a multi-frequency oscillator housed within the telephone instrument and generally powered by the line current.  When keying a number, the tones can be heard in the receiver, this is known as "confidence tone" and is intended to give the user an indication that the call is progressing satisfactorily.  MF signalling was first used in the UK on PABX's and is now also used on the modern electronic exchanges.  Below is a pictorial view of a UK standard 3 by 4 DTMF press button layout.  Run your mouse over the numbers to hear the MF tones.  Click here for a more detailed explanation .

Private Automatic Branch Exchanges are installed in customers premises.  They can be any sort of equipment ie Electronic, Crossbar, Strowger, as long as they switch calls automatically.  These exchanges are connected to a network and allow incoming calls to be connected to internal extensions and for extensions to make outgoing calls without the intervention of a person (i.e. operator) although an operator may be employed to route certain calls.  These systems also have internal facilities (divert, last number redial etc), just like modern day digital exchanges.

Party lines, in the UK, were where two customers shared one line.  They both had different numbers and the telephones rang independently of each other.  If both picked up their handsets at the same time, then both could hear the conversation.  Only one call could be made at any one time.  To make a call the user would press a button on the telephone to get dial tone.

Party lines were generally provided where there was lack of external line plant.  This was due to a large growth in new connections during the 1970's and early 1980's.  All the older telephone exchanges (Strowger, TXE and TXK) could cater for Party Lines, the modern electronic exchanges (System X and Y) could not.  Party Lines are therefore a thing of the past, although some people still share a cable pair, via a special device that allows for the lines to be independent from each other.

On early exchanges Party Lines also shared the same meter in the exchange.  This meant that one customer would be sent the bill and both parties would have to agree on how to split the bill.  Cause for many an argument and so the separate metering Party Line system was introduced.

Private Manual Branch Exchanges were installed in customers premises and were used to distribute calls to extensions, on the same site, by a person or persons.  Calls were extended by means of cords with plugs or switches.

This is the call transfer button used on telephones connected to telephone systems.  Pressing this button during a call allows the call to be transferred to another extension.  There are two types of transfer systems - Timed Break Recall and Earth Recall.

The name Strowger is used to describe mechanical step by step telephone systems.  The name originates from the "founder" of automated telephony, Mr Almon Strowger.  BT used to call Strowger exchanges, TXS.  All UK Strowger exchanges have now been recovered and replaced by Digital types.  Click here for more info

Processor controlled electronic digital telephone exchange used by BT throughout British Isles.  Designed and produced in collaboration with GEC and Plessey.

Processor controlled electronic digital telephone exchange used by BT throughout British Isles.  Made by Ericsson and also called Axe 10.

This type of recall is a timed disconnection of the line.  The recall button is connected to a timing circuit that automatically puts a predetermined disconnection to line.  The telephone exchange or telephone system sees this as a transfer signal.  On dial phones, dialling the digit 1 whilst in the middle of a call will emulate the this type of recall button.


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Last revised: May 20, 2022