The PAX—private automatic exchange—is an automatic telephone system purely for internal communication, with no connection with the outside world. Used in factories, offices, shops, cinemas, hotels hospitals and public authority buildings it provides fast and secret communication 24 hours a day without the need for operators or other human assistance. Introduced in Britain just before the first world war, many of these exchanges performed valuable service in munitions factories, naval establishments and the like.

The lack of outside-line facilities was not seen as a disadvantage: few staff needed to make or receive public network calls and those who did could be served by extensions off a manual switchboard (PMBX). Even after the concept of the PABX was born, which allowed one phone on a desk to be used for both internal and external calls, it was not generally welcomed.

To maintain control of expensive Post Office telephone costs, most firms preferred to restrict outside line facilities to a few key personnel and a manual switchboard ensured that time was not wasted on unnecessary chatter. This meant that a few staff had two instruments on their desk, one internal and the other external, but this was seen as no disadvantage (and because of the different ringing cadences there was no confusion between them when they rang). For a long time firms such as AEI promoted the two-telephone system, citing the advantage of instant internal communication on the PAX phone. The London Midland Region of British Rail concurred with this: when new PABX systems were installed at divisional offices during the 1960s, a 'house' PAX telephone system was also provided (the three digits of the PAX extension number were the same as last three of the four-digit number of the associated PABX extension).

Splendid isolation

Most PAXs served a single site and networked PAX systems were relatively unusual. PAXs could be interlinked using private circuits rented from the Post Office and later BT but the rental cost of these circuits tended to make this an expensive option. In London Lloyds Bank went for this option, having all minor branches of the bank linked to head office by a network of PAXs, as did the record store chain Soho Records. Public utilities could afford to installed networked PAX systems because they owned their own wayleaves, and many electricity boards had extensive PAX systems.

Since 1970 what was once seen as an advantage is now perceived as a weakness, and nearly all PAXs have been replaced by PABXs. This is not entirely due to the persuasiveness of PABX salesmen: most businesses can benefit from (and afford) a new fully-featured PABX and certainly the advanced enquiry and transfer facilities on these give extension users a flexibility they did not have on the old PABX 3 or 4. More importantly though, management attitudes and business methods have changed; many organisations work more efficiently and effectively if staff have full access to the outside world (rather than be isolated from it). Most other countries realised this many years previously and installed PABXs from the outset.

Endangered species

So now we know that the PAX is an endangered species and remaining examples are worthy of preservation. Because of their simple function their circuitry tends to be more straightforward than PABXs or public exchanges, and they were designed to work unattended without regular skilled maintenance. For practical use on museum sites and preserved transport facilities they are ideal; they are also handy for keen collectors of old telephones to demonstrate their collections as working exhibits. Not all PAXs were simple, though; most manufacturers offered a range of options for sophisticated customers. These included conference circuits, tie-line working, executive break-in on an existing conversation, revertive ringing codes to enable two telephones to share the same line pair (and still be able to ring each other) and code-call systems for staff location. With the last-mentioned users dialled an access code followed by the personal code number of a member of staff; after this a unique code of long and short bell or buzzer calls would be broadcast throughout a building.


Because of their simplicity, PAXs tended to show some idiosyncrasies. Some liberties were often taken with ringing signals and tones: they were normally single rather than double beat, and the ring tone was generally interrupted dial tone (or a feedback of the ringing current.) Tones were often produced by vibrating buzzers or valve oscillators (not used on the public network). The numbering schemes often showed no regard for Post Office practice: extension numbers could well start with the digits 1, 9 or 0, while on small systems you could well come across mixed 1, 2 and 3-digit extension numbers (1-9, 01-09 and 001-003). Extension is of course a misnomer, inasmuch as PAX telephones are not extensions of the public network. The Post Office more correctly called telephones stations on the few PAXs it installed, and for some reason (probably historical accident, with nobody having seen fit to remove it) a PAX dial label still figures in British Telecom's stationery stores catalogue.

It was stated above that PAXs had no access to the outside world; this was generally the case, though sometimes one PAX would be connected to another by means of a tie-line to enable two buildings or plants to be linked. Callers would dial the special tie-line code, wait for dial tone from the distant exchange and then dial the distant number required. Sometimes quite large numbers of PAXs were interlinked in this way, generally with one central exchange used as the tandem exchange or centre of the star-shaped network. Large hospital groups and electricity boards were typical users of PAX networks. Less commonly, a PAX might be interlinked to a PABX and occasionally astute users would find code combinations that would enable them to dial public network calls through the PABX.

No operator

Very few PAX systems had an operator as such—indeed, the whole concept of an automatic exchange was to do away with the need for an operator. That said, it was sometimes useful for users to be able to summon assistance with unknown numbers or receive urgent messages 'from the outside world'. For this reason, a PAX telephone was usually placed in the manual PBX switchroom, but this phone was an ordinary extension number, not a special code. Exceptionally, the London Electricity Board on its large PAX system covering the whole of London standardised on the code 222 for the switchboard at each exchange.

This situation meant that there was no need to reserve the code 0 for an operator. On some systems—particularly those supplied by GEC and Reliance and by ATE and Communication Systems—used 0 for the tie-line 'breakout' code. On Siemens Brothers systems the 0 level was normally taken to second selectors and codes such as 00 were used for tie-lines. Ericsson Telephones Ltd usually chose 9 for its tie-line code. Of course, on very large systems with a number of tie-line routes a number of single or two-digit codes would be used.

Manufacturers and contracts

Several telephone manufacturers set up separate divisions to deal with systems rented to customers. Thus Siemens Brothers had its PTD (Private Telephone Division), Reliance was the rental company in the GEC group, whilst Communication Systems was the rental side of ATE (later Plessey and then GPT, now entitled Siemens Communication Systems Ltd). Ericsson Telephones Ltd never had a rental division as such, although Telephone Rentals Ltd bought exclusively from Ericsson and had a close working relationship with Ericssons. For a time Reliance also used Budavox crossbar PAXs sourced from Hungary.

Not all customers could afford the prices that these firms charged; all these manufacturers did very well from Post Office public exchange contracts and were under no pressure to lower prices to win private system business. Accordingly a 'second division' of suppliers grew up, who sold simpler equipment at rather lower prices. Some of this equipment was made in Britain; a Wimbledon-based company called Autophone Ltd made exchanges and telephones for suppliers such as Dictograph Telephones Ltd and the British Home & Office Telephone Company (BHOTCo also had a subsidiary called the Birmingham Telephone Company).

Other companies supplied imported equipment. General Telephone Systems Ltd and Inter Office Telephones Ltd imported telephones and exchanges from Telefonbau und Normalzeit (T&N) in Germany, as did BHOTCo and a Leeds-based cmapny called Yorkshire Telephone Systems. Modern Telephones (Shipton Automation) took exchanges from DeTeWe (Germany) and telephones from Kapsch und Söhne (K&S) in Austria. All these were PAX systems, although around 1970 Modern Telephones acquired a licence from the Post Office to supply PABXs as well. BHOTCo also used T&N equipment at times, whilst some small independent installers used a variety of imported exchanges (e.g. Standard Electric from Portugal).

Some systems were sold to customers outright but far more were provided on a rental basis, normally on a seven-year contract. Typically these contracts would be renewed for a second seven-year period, after which the equipment would be considered life-expired and replaced. Old equipment would then be scrapped; suppliers went to great lengths to ensure old equipment did not filter back onto the market and normally declined any approach from customers who wished to purchase outdated equipment.



Installer Supplier
Automatic Internal Telephones Ltd
  • Siemens & Halske (Germany, pre-war),
  • Swiss equipment post-war
British Home & Office Telephone Co. (1)
  • T&N (Germany, pre- and post-war),
  • Autophone Ltd (Wimbledon) (2)
Centrum Communications (3) L.M. Ericsson (Sweden)
Communication Systems Ltd ATE Ltd.
Dictograph Telephones Ltd Autophone Ltd
General Telephone Systems Ltd T&N (Germany, post-war)
Hadley (Birmingham) Siemens & Halske (Germany, post-war)
Inter Office Telephones Ltd T&N (Germany, post-war)
Modern Telephones Ltd (4)
  • DeTeWe (Germany), K&S (Austria, phones only),
  • STC (PABXs and 7006 telephones)
Reliance Telephones Ltd GEC + Budavox (Hungary, crossbar exchanges only)
Shipton Automation
  • Tefag (Germany, pre-war),
  • DeTeWe (Germany, post-war)
Siemens Brothers Private

Telephone Department, later AEI

Siemens Brothers, later AEI
Telephone Rentals Ltd Ericsson Telephones Ltd, TMC Ltd.


1. applies also to their subsidiaries, the Birmingham Telephone Company and Emergency Warning Systems Ltd.

2. Autophone Ltd was related to BHOTCo.

3. Centrum belonged to Swedish Ericsson.

4. Associated with Shipton Ltd.

Who still uses PAXs?

Birmingham Airport had one (a modern electronic PAX) when we last looked and it is believed some power stations still have them. The number in use is declining rapidly, however. Another class of PAX user that may well still exist is the community PAX, of which there is (or was until recently) an example at Boulogne Maritime station and harbour complex. Operated by the local chamber of commerce, it enabled the railway and harbour offices, customs authorities and the various traders on the site to communicate without the expense of making calls on the public network. In the UK, Southampton Docks has or had a similar arrangement, although this is (or was) a PABX, not a PAX.


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