PABX No. 6

Descriptive Leaflet - 1966

Descriptive Leaflet - 1979

Introduced in 1966 - made by GEC.

Withdrawn by BT in 1993.

3 Internal
F 1660

Private Automatic Branch Exchange No. 6
(P.A.B.X. No. 6)


Scope of Instruction
This Instruction describes the facilities and equipment of the PABX 6.

Equipment Cabinet and Attendants Console
Attendants Console
(Answering Unit No. 1A with Telephone No. 710)

The PABX 6 is an unattended PABX, i.e. it is not attended by an operator, and incoming exchange calls are answered by certain designated extensions and transferred to the required extension.

It has capacity for 5 exchange lines, 20 extensions and 4 connecting circuits.  Two inter-switchboard circuits may be provided in place of two exchange lines, if required.

The Equipment, PABX 6 consists of a cabinet 3ft 9in wide, 5ft 4in high and 1ft 2in deep, which contains the basic equipment and wiring necessary to enable relay-sets, Uniselector mechanisms etc, to be fitted to give the required number of exchange lines, extensions, connecting circuits and inter-switchboard circuits.

pabx6ca.jpg (7770 bytes)
Equipment Cabinet Closed Equipment Cabinet with covers removed

The following general facilities are provided as standard. Further details are given in the relevant circuit Diagram Notes.

(a) Designated extensions
These are pre-determined extensions which are able to answer incoming exchange calls. Any extension may be so designated but the maximum number of designated extensions for which provision is normally made is eight. This number may be increased to twelve if necessary at the expense of night-service facilities. This reduces the number of possible night-service arrangements by the capacity of one night-service relay.

(b) Non-designated extensions
These are extensions which are unable to answer incoming exchange calls.

(c) Incoming exchange calls
An incoming exchange call rings common bells strategically placed in the building. When the handset of any designated extension is lifted the exchange call is automatically connected to it.

(d) Outgoing exchange calls
Access to an exchange line is obtained from an extension by pressing the telephone instrument button. For a short period during the setting-up of this connection a connecting circuit is employed, but if all the connecting circuits are busy, an alternative method of switching is used to ensure access to a free exchange line.

(e) Exchange call barring
Designated extensions may be barred outgoing exchange calls. Non-designated extensions may be barred incoming or outgoing exchange calls.

(f) Extension-to-extension calls
These are dialed direct using connecting circuits. The connecting circuit is held for the duration of the call. An extension may also dial another extension or a switchboard operator over an inter-switchboard circuit, dependent upon the type of P.B.X. at the distant end.

(g) Inquiry calls
While an extension is engaged on an incoming or outgoing exchange call, inquiry calls may be made to any other local extension, or, over an inter-switchboard circuit, to other PBX operators and extensions by pressing the instrument button and dialing the appropriate number. During the period of the inquiry the exchange line is held. On completion of the inquiry the original extension may return to the exchange call by re-pressing the instrument button.

(h) Transfer
An extension having made an inquiry call to a second extension while holding an exchange call may transfer the call to the second extension. This is done by asking the second extension to hold while the handset of the originating extension is replaced. Transfer does not take place if the inquiry has been made to an extension which is barred exchange calls, or to an extension or operator over an inter-switchboard private circuit. If for any reason transfer does not take place, the original extension is recalled automatically.

(j) Ring when free
If a designated extension makes an inquiry call and finds the required extension engaged, the designated extension will normally return to the exchange line and inform the subscriber. If the subscriber decides to hold, the designated extension may re-dial the required extension number followed by the additional digit 1 and then replace the receiver Provided that the required extension is not barred incoming exchange line calls, the exchange call is their 'parked' on the required extension line which is automatically rung when it cleats from the previous call. The exchange call is connected through when the handset on the required extension is lifted.

(k) Trunk offering
A designated extension wishing to transfer an exchange call to another extension may, on making an inquiry call to the extension, find that it is engaged. The designated extension may by dialing an extra digit 1 obtain access to the extension to offer the exchange call. During the time that the designated extension is intruding in this way on an established call a 'warn' tone is automatically applied to that call to indicate the intrusion.

(l) Night service
By operating a night service key which is located on an allocated designated extension telephone the normal extension conditions may be altered, e.g. extensions which are designated by day may be made non-designated by night or vice versa. Similarly extensions which are barred exchange calls by day may be given the facility by night or vice versa. It is also possible under night service conditions to arrange for additional or different call bells to be operated on incoming calls.

(m) Power failure
The power supplies are obtained from a mains-driven power unit. In the event of a failure of the supply the exchange lines are automatically switched to predetermined extensions which then function as normal exchange lines on the public exchange. Any calls which are in progress at the time of the failure are lost. On restoration of the power supply any call in progress from a predetermined extension is maintained until the call is completed.

(n) Inter-switchboard circuits
Two inter-switchboard circuits may be provided to give access to other PABX's or PMBX's over two routes or one route.

The total number of exchange lines which may be provided is reduced by the number of inter-switchboard circuits.

Access to an inter-switchboard circuit from an extension is obtained by dialing a code digit.

Tandem dialing over inter-switchboard circuits is not provided.

(o) Metering
When the PABX 6 is connected to a public exchange with S.T.D. and subscribers private metering facilities, total call meters may be provided for each exchange line and for extensions as required. Provision is made to accommodate these meters in the cabinet. The operation of these meters is dependent upon the application of 50 c/s pulses to the exchange line at the public exchange.

(p) Connection to exchanges
The PABX 6 may be connected to either Automatic, CB, CBS or Magneto exchanges.

(q) Attendant's telephone (optional)
The attendant's telephone consists of a modified Telephone No. 710 mounted on an Answering Unit No. 1A (grey).  This apparatus looks similar to a Planset No. 625 fully assembled.


An Article from
Post Office telecommunications Journal
Winter 1965
By A. H . Hearnden


New unattended small private automatic branch exchange which dispenses with the need for the services of an operator and allows incoming exchange calls to be answered at extension points and re-directed, as required will soon become generally available. It is the PABX 6, an improved version of the PABX 5 which was introduced in 1963.

The PABX 6 has a maximum capacity of 20 extensions and five exchange lines.  Two inter-PBX lines can be terminated in place of two of the exchange lines.  The equipment, including the mains power unit and ringing converter, is contained in an elephant-grey steel cabinet 3ft 9ins wide, 5ft 4ins high, and 1ft 2ins deep.  The switching equipment employs Post Offiice standard components such as relays and uniselectors which act as linefinders and final selectors.

The extension numbering range is 20-29 and 30-39.  Extensions are classified as “designated,” that is able to answer incoming calls, or “non-designated” and may be allowed or barred outgoing exchange line calls.  Non-designated extensions may also be allowed or barred transferred incoming exchange line calls.  Calls between extensions have first party release.

To make an out going exchange call the extension user presses the button on the telephone to obtain the public exchange dialing tone. Connections to other PBX’s are made via the interconnecting private wire lines by first dialing a single routing digit. In the absence of an operator, an extension which has answered an incoming call must be able to hold it for enquiry to another extension and to transfer the call if necessary. Enquiry access is open to all extensions (including barred extensions) or to an extension or operator over an inter-switchboard circuit.

To make an enquiry call the telephone button is depressed and the appropriate extension or inter-switchboard line number is dialed.  A holding condition is applied to the exchange line during the enquiry and by depressing the button again, the original extension can return to the exchange line at the end of the enquiry.

Transfer of the exchange call to the second extension is effected by simply replacing the handset on the attending telephone. If the second extension is busy when an attempt is made to transfer a call, intrusion is possible by dialing a further digit “1” which overrides the busy condition.

Another facility offered by the PABX 6 is that an incoming call can be parked on an engaged extension which, having previously been offered the call, will be re-rung as soon as the call in progress is finished.  During the waiting period no tone is returned to the caller and no indication is given to the extension that another call is waiting.

The provision of night service on an unattended PABX differs from that on other PABX’s in that it is essentially a method of altering the classification of extensions to suit night staffing arrangements.  This is achieved by a night service key fitted on one designated extension which alters the designation and barring condition of the other extensions as required. Additional bells can be added for night service.  If the mains supply fails each exchange line is diverted to a predetermined extension which the functions as a direct exchange connection.

Since it may fall to the lot of a typist or secretary to answer the bulk of incoming calls on a unattended PABX a special attendant’s telephone has been designed to cater for this requirement.  The telephone concentrates two designated extensions at one station and provides facilities for holding a call on either line.  During busy period the user can handle incoming traffic without having to have two designated extension  instruments on her desk.

GPO Picture of the PABX No. 6

This is a standard PABX No. 6 with no meters


GEC Pictures of the PABX No. 6

Front view with doors on
To remove the doors - the lockable bar between the meter housing and the doors, is firstly removed - then the two doors are lifted up and then away from the cabinet
Front view with doors removed
Note the meters at the top - the left meters are for extension metering whilst the right hand meters are for exchange lines.
The Power Supply is lower left and the ringing generator is middle left.
Rear view


Front view with some relayset covers removed
Close up of the terminating strips - these are behind the uniselectors
View of PABX No. 6 production at the GEC Coventry Works
Note the soldering irons in ovens, to keep them hot (centre of picture)
View of PABX No. 6 production at the GEC Coventry Works
View of PABX No. 6 production at the GEC Coventry Works


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Last revised: June 08, 2022