MF press button, table telephone.

Field trial item - similar looking to Telephone No. 726.  The pictures show the internal arrangements.  The rear view shows the inductor coils used for signalling purposes.

Used in Langham Exchange field trial with the first press button telephone being installed on the 10th January 1966.

t726.gif (11298 bytes)

GPO Press & Broadcast Notice

6th December, 1965


Press-button telephones are to be put on trial for three months among nearly 300 subscribers on the Langham telephone exchange in London early next year.

The press-button telephone has ten buttons instead of the ten finger holes in the dial of the ordinary telephone.  On the experimental telephones being used for the trial the buttons are arranged in two rows of five:-

1 2 3 4 5
6 7 8 9 0

but future standard instruments are likely to have them in the form:-

1 2 3
4 5 6
7 8 9

Two different experimental systems will be put on trial at Langham exchange.  One uses voice frequency tone signalling generators inside the telephone working into special equipment in the exchange.

In the other system the press-button telephone is connected to operated convertor, which can be placed under the table or and does not need any special equipment at the exchange.  This can be used therefore, for internal calls on existing types of private exchange, as well as to other numbers on the public network.

Press-button telephones enable the user to complete his "dialling" action much more quickly - as quickly in fact as he can press the buttons.  This not only saves some time, but will enable all-figure numbers to be keyed with less chance of error.  The time saving in setting up the call will be greatly increased on the fast switching electronic exchanges of the future.  Press-button telephones being put on a further trial at the new electronic exchange opening at Leighton Buzzard next year will enable calls to other Leighton Buzzard numbers to be completed as fast as the buttons can be pressed.

Technical data from the trials will assist in the design of standard press-button telephone systems that may be offered as an optional facility in the future and assessment of their popularity will help in working out possible costs and charges.

In the field of data transmission press-button techniques offer the possibility of developing new methods of feeding simple data into a computer over telephone lines.

If all goes well there may be a big opportunity for exporting press-button telephones.  The Post Office has, therefore, a double interest in the success of the trials.  First to pioneer a system that will help subscribers, and second to boost British exports of an advanced technological kind.

An article from
Post Office Telecommunications Journal
Spring 1966 (Page 30)
By H. W. Jose

Many novel items of telephone apparatus will be needed to meet the rapidly-advancing requirements of a technological society.  Market research is one of the instruments used by management to anticipate subscribers' needs and to examine and appraise new ideas and prototypes, among them ..


Automatic switching systems faster than the present Strowger system are becoming available.  The cross-bar system is one and the electronic exchanges, now being developed for the Post Office in collaboration with the British telecommunications industry, are designed to reduce running costs and, what may be equally important in the long run, to set up a call in a few thousandths of a second.

Right: One of the new press-button telephones now on trial by Langham subscribers.  Some subscribers on the new electronic exchange soon to be opened at Leighton Buzzard will also try out a number of experimental PBTs.

For these and other economic reasons it is highly probable that within the next few years a considerable number of new exchanges will be electronic and that electronic equipment will start to be used for exchange extensions and replacements.  In the next few decades the whole network will probably be based on electronic terminal and transit switching equipment.

If these high-speed electronic exchanges are to achieve maximum efficiency they must be complemented by a "dial" which can operate as fast as is humanly possible.  It is impossible for any person to beat the speed of an electronic exchange.

A possible solution is the press-button telephone which has 10 buttons or keys that can be pressed one after the other without pause.  But how would subscribers react to such a fundamental change in the method of using a telephone?  Would they reject it as unmanageable, accept it reluctantly or welcome it to the extent that they would be willing to pay extra for it (as they have with the modern coloured telephone)?  Also, would the general introduction of press-button telephones effect any significant change in the "dialling" error rate?  It would be foolish to introduce in large numbers a new telephone which subscribers did not like and which degraded, even marginally, the quality of service.

The first wholly electronic public exchange is planned to open later this year at Leighton Buzzard, Buckinghamshire, where some subscribers will be given an opportunity to try out a small number of experimental press-button telephones.  The combined advantages of high-speed keying and exchange switching will be apparent only on local calls and it will be well into 1967 before the results of even these limited trials are available. Meanwhile, orders for other electronic exchanges have had to be placed well in advance and it needs to be known as soon as possible whether or not press-button telephones are to be provided and, if so, how many.

Market research into press-button telephones started in 1962 with the installation on Post Office Headquarters' automatic (Strowger type) exchange of a special unit to accommodate 24 press-button telephones.  Each telephone contained two small tone generators (about the size of small cotton reels), each of which sent one of four tones to line whenever a button was pressed.  The special unit at the exchange identified these tone signals, stored the information at high speed and then regenerated the signals in the form of the slow-motion pulses needed to operate the standard exchange equipment.  This system of signalling is known as Voice Frequency Press-Button Telephone (VF PBT).

Staff in non-technical departments at Headquarters were specially included in the trial and all users said after some weeks of operation that they preferred the press-button telephone to the usual dialling system and experienced fewer wrong numbers.  Remote service observations were not possible.  A significant feature of these first trials was the way they highlighted the speed with which numbers could be keyed and the comparatively long time subsequently taken by the standard exchange equipment to process the information.  Most of these telephones are still in use today and those who have them would give them up with some reluctance.

Post Office Fitter Arthur Aylott installs the first press-button telephone to be brought into public service. This is the special register equipment installed at Langham Exchange for converting from press-button to Strowger dialling.

The results of these trials, together with information about developments overseas, led to the idea that there might be an application for press-button telephones on existing Strowger-type exchanges.  Indeed, it was foreseen that if PBTs were made available later at electronic exchanges, subscribers on Strowger-type exchanges would want them despite the delay between keying a number and getting ringing tone.  To test this theory and estimate the numbers likely to be ordered, it was decided in 1964 to order 300 PBTs and use them in one exchange area to test subscribers' reactions -  particularly their willingness to pay extra for this novel facility - and to assess the effect of PBTs on the "dialling" error rate and on the service generally.

Left, the front and right, the reverse sides of a power and miscellaneous panel of a press-
button telephone multi-frequency receiver

However, the Voice Frequency Press-Button Telephone needs to he directly and permanently connected over an exchange line to the special equipment at the exchange and this ruled out those business subscribers who might want PBTs on extensions served by existing private automatic branch exchanges (PABXs).  On PABXs, VF PBTs could not be used to set up internal calls or get access to an outgoing exchange line.  Other problems were associated with PMBXs and particular plan extension arrangements.  To provide PBT facilities for subscribers on these installations it was decided to produce a self-contained instrument which comprised a mains-operated storage and regenerative device in a "black box" which could be installed beneath the subscriber's desk or in another unobtrusive position near the PBT.  Without this form of press-button telephone it would not be possible to explore properly the reactions of all classes of subscriber.

Technical Officer W. T. Fitzgerald operates a register testing equipment with "digitron" display at the Langham Exchange. The special register identifies the tone signals, stores the information at high speed and regenerates the signals in the form of slow-motion pulses needed to operate the standard exchange equipment.

The exchange chosen for the trials of these two experimental systems was Langham, in central London.  Langham has a good mixture of business and high-income residential subscribers; it is close enough to the Headquarters' engineering and administrative units to ensure on-the-spot attention, and there was enough space in the exchange building to erect the new racks and special equipment needed.  Contracts for the apparatus and equipment were placed in 1964 with General Electric Company Ltd. and Ericsson Telephone Ltd.

The first PBT brought into public service in the Langham area was installed on 10 January, 1966, by Post Officer Fitter Arthur Aylott in the office of Mr. F. Scott Matthews, Chairman and Managing Director of Colgate-Palmolive Ltd., in Oxford Street.  Before this, however, the Post Office sent illustrated postal questionnaires to a random sample of nearly 400 Langham subscribers.  These questionnaires were despatched before it became public knowledge that the trials were to be held.  Suitable questions were included to find out how interested subscribers were in Trimphone and automatic dialling aids as well as PBTs, thus screening the main purpose.  About 160 respondents said that they were interested in PBTs but of these only 100 (about 25 per cent) were sufficiently interested to want to know when, if at all, PBTs would become available.  These 100 subscribers formed the nucleus of the trial and the balance of 200 was made up by more random sampling of Langham subscribers who were informed at the outset that the Post Office needed people to help test the experimental PBTs.

An essential feature of the trials is that for the first three months the equipment is provided free of charge.  This gives the Post Office and the customer a fair opportunity to assess all aspects of the new equipment.  At the end of the free trial and if the Post Office decides to keep the experimental systems in operation, the subscribers will be given an option to retain their PBT for an extra rental or have their old installation reinstated.  The outcome of these offers will be an important factor in determining future marketing policy for PBTs on both Strowger and electronic exchanges.

The problem of relating the results of the Langham, and later the Leighton Buzzard, market trials with national requirements has already been explored.  One step towards a possible solution is the despatch to a random sample of subscribers all over the country of a postal questionnaire similar to that sent at the outset to the Langham subscribers.  It may then be possible to compare the attitudes in Langham with the attitudes over the whole country and make some assumptions regarding the final outcome in Langham with the likely outcome if press-button telephones are introduced nationally.

Additional information

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