PABX No. 4

Whilst the BPO maintained the large PABX's, they were never allowed to make or install them.

Customers would purchase a PABX from the manufacturer of their choice who would design and install a system to suit the customers needs.  The BPO would oversee the installation and then take on the maintenance responsibility for the equipment.

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Click here for Plessey PABX No. 4

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Private Automatic Branch Exchange No. 4
(P.A.B.X. No. 4)


The PABX 4 has virtually an unlimited capacity and uses the same standard types of automatic equipment, power plant and extension telephones as the PABX 3. The capital cost - which must be met by the customer - is greater than that of a PABX 3 of the same size, but it offers more facilities and uses a desk-type, cordless, manual switchboard in contemporary styling.

It incorporates several novel features which will, if fully utilised, produce a marginal saving in operator time, but with some loss of personal service to the extension users. The differences in facilities and degree of personal service between the PABX 3 and 4 need careful assessment and consideration before a choice is made. The Telephone Manager will gladly help and advise on the relative merits of the two systems in particular circumstances.

As with the PABX 3, the customer must buy the equipment from an approved contractor who will also be responsible for installing it. The Post Office provides the extension wiring and telephones, and maintains the whole installation.

Apart from the operator’s switchboard the PABX 4 is similar in design and construction to the PABX 3 already described; the automatic apparatus is mounted on the same type of open racks, and a similar type of power plant is provided. The main difference is the provision of the cordless, desk-type switchboard, and for this reason the PABX 4 is often called the cordless PABX. Dependent automatic installations in separate premises can be associated with a PABX 4, as with a PABX 3.

In common with the other systems described in this booklet, the PABX 4 provides a normal range of PABX facilities, including extension-to-extension dialling; extension-to-exchange dialling; connexion of incoming exchange calls by the PABX operator; through clearing; hold for enquiry; operator call-in for assistance; automatic transfer of calls; and night-service arrangements.

Some special features are:-

  • Selected extensions can be given priority calling to ensure that their calls are dealt with by the operator before calls from other extensions.
  • Incoming calls are put through to extensions by the PABX operator by the use of press-buttons. If a called extension is engaged the operator can leave the incoming call to be connected automatically when the extension is free.
  • The operator can speak to an exchange-line caller or to the called extension without being overheard by the other person.
  • Selective restriction on calls can be applied so that particular extensions can be prevented from making and/or receiving exchange calls, or having exchange calls transferred to them from other extensions. Inter-PBX line barring can also be arranged.
  • Facilities are usually provided for the operator to use press-buttons instead of a dial for outgoing exchange calls.
  • Calls over inter-PBX lines from other switchboards may be received by the PABX operator or, when signalling conditions permit, may be received direct by the extension required. Conversely, in most cases extensions can dial directly to other switchboards, and sometimes - if the distant switchboard is another PABX - directly to the required extension.

Night-service can be provided in one of the following ways:

  1. Incoming calls on exchange or inter-PBX lines cause suitably sited bells to ring continuously until any extension answers by lifting the handset and dialling 8. If necessary, the call can be transferred to another extension in the usual way.
  2. Incoming calls ring the bells of certain designated extensions, any of which answer simply by lifting the handset. If all designated extensions are engaged the next incoming call may ring an overflow bell or bells, or inject a warning tone on to a preselected designated extension. All designated extensions retain normal extension facilities.
  3. Each exchange line on which night service is required is connected to an extension in such a way that the extension may answer all incoming calls on that line while retaining all normal extension facilities. A warning tone is given on any such extension which is engaged on an internal call when an incoming exchange call arrives, and when the handset is replaced the extension is re-rung automatically.
  4. Each exchange line on which night service is to be provided is connected to a nominated extension in such a way that the extension becomes equivalent to a direct exchange line and loses all extension facilities. Only one extension can be connected to each exchange line.


One or more positions are provided, depending upon the size of the PABX. They are cordless, desk-type, floor-standing, contemporary in appearance, with compact styling, and adaptable for use in any shape of room since they do not have to be assembled in line as a suite. Maximum use is made of automatic techniques to reduce the amount of manual operating work.

The switchboard has a small desk with a sloping face panel on which are press-buttons, switches and either display lights or a translucent screen. Each approved contractor may produce his own version of the manual switchboard and although these conform to certain basic requirements, they may differ considerably in appearance. When making an outgoing call from the PABX switchboard the operator is automatically connected to a free exchange line by pressing a button. The call is set up through the automatic equipment and when established is disconnected from the switchboard. The PABX is designed to meet the needs of offices where extensions are allowed to dial their own exchange calls, and it is not as easy for the operator to obtain them as it is on a PABX 3: the operator cannot connect an out going call, on behalf of an extension, until the extension handset has been replaced, and the extension therefore has to be rung back.

Presentation of calls to the switchboard
All calls to the switchboard are presented approximately in their order of arrival to operators. Under one system there are separate lights to indicate an exchange call, a call from an extension, a call from another PBX, etc, and if more than one light shines operators can select which type of call to answer. In another system all incoming calls share the same light and the type of incoming call is indicated by another light after the operator has answered the call.

Numbering arrangements and dialling codes
These are similar to those used on the PABX 3 except that 8 is usually reserved for night service.

These are similar to those on the PABX 3.

Standard dialling, ringing, engaged and number unobtainable tones are used and, in addition, there is a ticking tone which indicates that the operator has come in on an engaged line. Exceptionally, a distinctive ringing tone may be used for internal calls if required.

The same considerations apply to PABX 4 accommodation as for the PABX 3. There is some saving in the size of the room needed for a multi-position switchboard because the cordless boards can take up to 30 per cent less floor space than that needed for a similar number of PABX 3 switchboard positions, but the automatic equipment will usually need more room. A table showing typical room sizes is given overleaf.

Optional Facilities
As with the other types of PABX there are various optional facilities that can be readily provided. Some of these are
Conference telephone calls between up to ten extensions, or between one exchange line and up to five extensions.

Key callers to enable selected extensions to call up to twenty pre-determined extensions by pressing the appropriate key.

Staff location systems under the control of the operator. Call queuing for calls to busy extensions. Reception of incoming exchange line calls direct on to selected extensions. Remote controlled dictation systems.


Typical room sizes for PABX 3 and PABX 4 installations
The dimensions shown are intended to be a guide only; the sizes quoted are approximate and some adjustment may be necessary following a more detailed investigation of requirements.

Number of extensions
Not exceeding
Dimensions of switchroom (ft) Dimensions of 
equipment room (ft)
Dimensions of battery room (ft) Total
floor area (sq. ft)
PABX 3        
100 extensions with 2 positions 12 x 11 14 x 13 - 314
200 extensions with 3 positions 14 x 11 16 x 14 - 378
400 extensions with 4 positions 16 x 11 22 x 18 - 572
500 extensions with 4 positions 16 x 11 24 x 18 13 x 6 686
600 extensions with 5 positions 18 x 11 25 x 22 14 x 6 832
800 extensions with 7 positions 23 x 14 27 x 26 16 x 6 1120
1200 extensions with 10 positions 29 x 14 32 x 27 17 x 6 1372
PABX 4        
100 extensions with 1 position 12 x 10 22 x 14 - 428
200 extensions with 2 positions 15 x l0 24 x 18 - 582
400 extensions with 3 positions 18 x l0 28 x 2l - 768
500 extensions with 4 positions 21 x 10 26 x 24 19 x 6 948
600 extensions with 4 positions 21 x 10 26 x 26 19 x 8 1038
800 extensions with 5 positions 24 x 10 31 x 26 19 x 8 1198
1200 extensions with 6 positions 27 x 12 43 x 36 28 x 8 2096

 Taken from GPO Descriptive Leaflet DLD 400 (1/68)

Article taken from
Post Office Telecommunications Journal
Winter 1966

Introducing . . . .

By P. A. Marchant

A standard system has been introduced for the PABX 4 to overcome the problems created by the need to deal with a number of variants.

Six types of Private Automatic Branch Exchange are at present in use by the Post Office.  They are the PABX 1 (with a cordless switchboard and automatic equipment for a maximum of 49 extensions); the PABX 2 (a similar equipment but with a cord switchboard); the PABX 3 (automatic equipment with a cord manual board for large installations); the PABX 4 (a cordless type also for large installations); and the PABX 5 and 6 (small, fully-automatic systems catering for a maximum of 20 extensions and which need no switchboard operator).

All these PABXs are of standard design except the PABX 4 which, in the past, has been supplied by using manufacturers’ own proprietary designs. Although design approval by the Post Office has ensured that acceptable equipment and operational practices have been used, inevitably differences exist between them.

To overcome the problems arising from the need to deal with five or more variants, a standard system has now been introduced. It will be known as the PABX 4, the previous models being distinguished by the manufacturers’ prefix - for example PABX STC 4.  To provide the facilities required of a modern PABX, the switching system of the PABX 4 is necessarily complex and can be only briefly described here.

The design follows conventional step-by-step switching principles but some novel features have been included. The equipment is extensible from the basic unit of 50 extensions to a total of some 7,000 extensions, with no limitations on the number of manual positions, exchange lines and private circuits which can be provided.

The basic design covers all the facilities normally required by a PABX customer.  In addition, special facilities can be provided although these are not all as yet covered by the standard design and will continue to be supplied in a proprietary form for the time being.  Equipment practice also follows the conventional pattern.

Satellite PABXs can give considerable line economy and they have been catered for in the new design, full facilities for satellite extensions being achieved mainly by the addition of small registers at the satellite.  This, with other design features, ensures that an extension connected to a satellite now enjoys the same facilities as it would if connected to the main PABX. In addition, there is no need for special routing codes to and from the satellite or for different operating instructions.

Although the objective of a cordless system is to reduce operator control of calls to a minimum, the switchboard still retains a position of importance in the system.  Telephonically it is the shop window of the PABX. From a design aspect, cordless switchboards have the advantage that functional requirements are no longer the major consideration.  Operators are not now required to sit closely in line facing a formidable array of multiple jacks. Instead, they can sit at attractive desk-type cordless positions with simple press-button controls with an illuminated display of operating information presented automatically as required.

Switchboard presentation is important and variety in design, which allows customers a choice, is a distinct advantage.  This has been achieved in the PABX 4 by giving the manufacturer freedom in design while providing the essential operational and engineering requirements of the Post Office.  All the switchboards have been approved by the Council of Industrial Design and constitute the standard range offered by the manufacturers.  Other designs which may be offered to suit special requirements are not excluded.

The functional part of a cordless position is little more than an assembly of keys and lamps used to control and supervise the automatic equipment.  The switchboard provides no through connection for any call.  Controls are routed from the automatic equipment to the switchboard as and when the operator calls for them.  The presentation of calls to the operator, the routing of calls from the operator and the final clear down of connections are all performed automatically.  This simplification of operator control and supervision is achieved by adding automatic equipment to perform the functions previously completed manually.

The cost of this additional equipment can be offset if the facilities provided by the cordless system are fully used to allow savings in switchboard equipment and operating staff.

The two main components of the switchboard are the display screen and the control keyboard. By means of an illuminated display, the former tells the operator of calls awaiting answer, the class of call, route information, line identification, supervision of calls in progress and provides other miscellaneous information.  This information is displayed by lamps mounted at the rear of a non-reflective glass screen, with a suitably marked negative film interposed between lamps and screen.

All keys on the keyboard are of the press button type.  Located centrally are the connecting circuits on which calls are held while being progressed by the operator.  Normally six are provided, the maximum being eight.  On the left of the keyboard are call selection and miscellaneous control keys and on the right a keysender for use on both internal and external calls. The dial is for emergency use only and is normally hidden from view.

Incoming calls to the PABX 4 may be operator-connected or dialled-in directly to the extensions. Normally, all incoming exchange calls are operator-connected as are inter-PBX calls if they cannot be given dialling facilities.  Where dialling is provided the arrangements follow standard PABX practice.

When an incoming call arrives for connection by an operator a signal is given on all cordless positions. In all circumstances an indication must be given of the class of call, that is, whether it is an exchange, inter-PBX or an assistance call.  Common answering may be provided which means that the next call waiting, whatever its class, is accepted by the operator.  Alternatively, the operator can select the class of call to be answered.  The operator accepts incoming exchange calls by depressing an answer bar or key and selecting a free connecting circuit. She then routes the caller automatically to the extension required, by using the keysender.

When the call is established all switchboard contact is automatically released from the connection.  Up to this point full supervision is given to the operator.  lf for any reason the call has to be supervised the hold key of the connecting circuit concerned may be operated to prevent automatic release from the switchboard.  lf a called extension is engaged and the caller decides to wait, the call remains connected under operator supervision until the extension becomes free when it is automatically rung and connected.

With minor exceptions, outgoing calls from the PABX can be made automatically by the extensions.  This is the ideal arrangement if maximum advantage is to be obtained from a PABX and particularly so with cordless working.  Unfortunately this ideal is not always obtained in practice mainly because freedom of access to the outside network for all extensions does not always suit the customer.

Where outgoing traffic is dealt with automatically the arrangements follow normal PABX practice.  The extension dials the appropriate routing digit or digits to obtain connection to the outside lines. If a call is to be connected by way of the manual position “0” is dialled.  The operator accepts the demand and connection is made by reversing the call to the extension concerned.  When established, the connection is identical to that for an incoming call.

Apart from extension-to-extension connections, all established calls can be held while an enquiry is made of another extension or transferred to another extension under the control of the extension user.  This is a common feature of cordless systems where a high degree of extension control of calls without operator intervention is desirable.  The operator may, however, be called in if desired and there is a built-in safeguard which ensures that in the event of misoperation by the user an outside caller is not left unconnected but is switched automatically to the operator.

Another important facility which can be readily applied to large cordless systems and which has been designed into the PABX 4, is direct dialling into PABX extensions from the main exchange network (DDI).  This is achieved by allocating exchange numbers to the extensions, so that a call is routed from the main exchange through the PABX equipment directly to the required extension without the intervention of the PABX operator.  This facility will undoubtedly become popular when exchange multiple numbers become more readily available.  When fully used it could remove the only remaining essential switching junction of the cordless switchboard operator and thereby reduce the operating responsibilities to assistance and enquiry only.

With STD in mind, the PABX 4 provides an operational facility for metering calls at the switchboard on a common exchange line basis, or at the extension point.  The switchboard provides for meters at the right of the display panel which the operator can associate with any outgoing call set up by way of the switchboard. Association of the meter is automatic, STD codes may be barred to individual extensions and, as with all PABXs, access to the exchange network can be barred completely.

Tandem switching for private circuit networks, either manually or automatically operated, can be provided and among the many other facilities are full night-service arrangements, access to dictation systems, supervisors and enquiry desks.  A standard system of keycalling, which gives immediate access between an executive and a selected group of extensions and can incorporate loud-speaking telephones, will be available shortly.

The optional features which have been provided for a number of cordless installations in the past will continue to be available, although the frequency of demand may not in every instance warrant the development of standard arrangements.  The optional features include conference facilities set up by the operator or controlled from selected extensions; emergency call arrangements resembling an internal 999 Service; manual extensions giving immediate access to the operator; short-code dialling (obtaining frequently called numbers by dialling or keying a code of two or three digits); night watchman’s patrol systems; staff location systems; internal queuing systems for enquiry bureaux; calling line identification to the operator; special facilities for hotels and the provision of automatic accounting of calls; and press-button telephone working.

This list is by no means exhaustive.  The new PABX is expected to be closely integrated with the needs of industry and commerce and not necessarily confined to precise telephone communication.  No doubt the future will call for even more elastic treatment but the PABX 4 equipment should be able to cope.

Click for an article from the Ericsson Bulletin on the ET P.A.B.X 4

Pictures of Ericsson installed PABX 4 Consoles in the Fleet Building (Dated 1962)



Pictures of Ericsson installed PABX 4 Consoles (Dated 1966)




Pictures of the Ericsson Telephones ET4

Operators positions with two enquiry desks on the left (Ericsson ET4 type)

Apparatus racks (Ericsson ET4 type)

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Last revised: January 18, 2022