Numbering Schemes in Automatic Telephone Exchanges
by I. W. Whiston
From Traffic, volume 11, no. 3 1966/67.
When the opening of S.T.D. was being planned it was necessary to design a numbering scheme for the country which would last for all time. It seems, therefore, that our national numbering scheme has reached its final form and that the next step will be an international numbering scheme. It may, therefore, be of interest to look back and see how our numbering scheme grew. In the early days of automatic exchanges there were not the technical restrictions which there are at present with a highly developed automatic system and attempts were made to restrict the number of numerical digits which a subscriber had to remember. With this aim in view, various schemes were considered using a combination of letters and numericals.
The development of numbering schemes for automatic exchanges in this country may be divided into six stages:
- (1) The very earliest automatic exchanges, the incunabula exchanges, viz. Official Switch (O.S.) and Epsom.
- (2) The decade 1914-1924a period when mechanization was slow because of the war and several different automatic systems were given a trial.
- (3) The period from 1924 until just after the last war. During this period Director working was introduced in London, Birmingham and Glasgow, Liverpool, Manchester and Edinburgh and the standard for other systems was a numbering scheme not exceeding five digits.
- (4) The introduction in the early 1950s of 6-digit numbering schemes.
- (5) Introduction of the national numbering scheme in 1958.
- (6) Introduction of all-figure numbering in 1966.
Stage 1. The first automatic exchange to be opened in this country was Epsom on 18th May, 1912, with 381 lines and 443 stations. Epsom was chosen as it was close enough to London for its performance to be watched by the Administration and the Engineer-in-Chiefs staff. The equipment selected, after several different systems had been examined, was the Strowger type and was purchased from the American Automatic Company through its British agents, British Insulated and Helsby Cables Ltd. The cost quoted by the makers was £4,258 6s. 3d. for the automatic equipment and £1,212 19s. 1d. for subscribers apparatus. They quoted:
- Time required for start of work after receipt of order ... ... ... 50 days
- Time required for completion of work 45 days
- TOTAL 95 days
One of the disadvantages of choosing an American system was that it had no multi-metering facilities. The General Manager of the London Telephone Department had pointed out that multi-metering would be unnecessary and that ticketing of calls could be abolished if a common charge was adopted for local and junction calls and, although this suggestion was made several times in later years, it was not adopted until 1958 with group charging.
Two months after the opening of Epsom exchange, equipment of a similar kind was brought into operation to serve Post Office Headquarters departments. It was known as Official Switch and was installed in G.P.O. (West) with 320 lines and 418 stations.
There was also a proposal at this time to have a trial automatic exchange at Portsmouth, but the Treasury would not agree.
Details of the numbering scheme of Official Switch have not survived, but it was probably similar to that adopted for Epsom which was as shown below:
- 0 Operator (at Epsom, for trunk and junction calls)
- 9 Test Desk
- 8 Information and complaints
- 7 }Subscribers lines 200-799
- 6 }Subscribers lines 200-799
- 5 }Subscribers lines 200-799
- 4 } Subscribers lines 200-799
- 3 }Subscribers lines 200-799
- 2 }Subscribers lines 200-799
- 1910-1929 Coinbox lines
- 17 Sutton exchange
- 16 Croydon exchange
- 15 London Central exchange
Level 0. At Epsom the code 0 was used for calling the operator, a practice imported from the U.S.A. and eventually adopted as standard. The argument for its use being that if numbers beginning with 0 were used, subscribers might regard nought as standing for nothing and omit dialling the digit. Fortuitously, when lettered dials were introduced, 0 had the advantage of being the initial letter of Operator.
Level 1. When Epsom was designed it was intended to use level 1 for subscribers numbers but, shortly before the opening, this was abandoned because it was expected to give trouble. This suspicion about the reliability of level 1 for subscribers numbers, which originated at Epsom, lived on for a very long time in spite of suggestions over a number of years to make use of the level in various ways, and it remained under a cloud from which it did not emerge until a few years ago when it was decided to use it for service codes.
Stage 2. The second stage covered the period up to 1924 when several different types of automatic systems were given a trial. In 1914 two automatic exchanges were opened, Hereford and Darlington. At Hereford the Lorimer system, made by the Canadian Machine Telephone Coy. was used. It had many unusual features, meters at customers premises, a key-caller built into the telephone instrument and multi-metering for two unit fees. The equipment was originally ordered for Caterham Valley exchange but, because of delay in delivery, the equipment was transferred to Hereford. This had the advantage that the multi-metering facility could be used; at Caterham Valley it was not required. Darlington exchange was fitted with the Western Electric Rotary system. The numbering schemes were:
- 1000-1999 Subscribers lines
- 1400-6 Operators
- 1410-11 Phonograms
- 2 }
- 3 }
- 4 } 5-party line circuits
- 5 }
- 6 }
- 7 } 2-party line circuits
- 8 } 2-party line circuits
- 0000 }
- 9999 } Trouble lines *
- 1000 }
*These were not allotted, as fault conditions would have the effect of calling these numbers.
- 12 Trunk and junction calls
- 14 Complaints and enquiries
- 2600 Phonograms (final selector numbers)
- 2000-2799 Subscribers lines
The next exchange to be opened was Accrington on the 6th March, 1915. Here 0 was used for the operator for trunk and junction calls, 9 for enquiries and 7 for the test desk. Other automatic exchanges opened in this period, had the same numbering scheme with level 8 being used for rural party lines, and for phonograms a final selector number or a code was used. The Accrington numbering scheme was:
- 0 Operator (at Blackburn)
- 9 Enquiries
- 8 Spare
- 7 Test Desk
- 6 }
- 5 }
- 4 } Spare
- 3 }
- 2000-2799 Subscribers lines
Leeds, opened on the 18th May, 1918, was the first exchange designed for a large city, and more attention was given to the allocation of levels; 90 was used for phonograms and 92 for official numbers. The numbering schemes for this exchange had all the main features which were in general use until the recent changes in the use of levels 1 and 9 when group charging was introduced. The numbering scheme was:
- 0 Operator (for trunk and junction calls)
- 90 Phonograms
- 99 }
- 98 } Test desk
- 97 }
- 92 Official numbers
- 8 }
- 7 }
- 6 }
- 5 } Dead levels
- 4 }
- 3 }
- 2 Subscribers, coinbox and party lines
Stage 3. At the end of the first world war all but a few of the telephone exchanges in large cities, including London, were manual, and an accelerated programme of conversion to automatic working was planned.The economics and advantages of automatic working were, however, not yet generally accepted. Nevertheless, the climate was very much different from 1912 when the view was expressed that automatic working was economic only where nearly all the calls between subscribers on the same exchange, and the P.M.G. had said that he would not contemplate the introduction of automatic apparatus at exchanges, like those in London, where 80 or 90% of the calls were for other exchanges. Nor at that time were the Americans in favour of automatic working. The President of the American Telephone and Telegraph Company said that he did not believe that automatic working would ever replace the manual. Automatic working is not adapted to our needs, any more than radiography will ever supplant telegraphy by wires.
This is a complete list of exchanges opened in this stage.
London. The choice of a suitable automatic system and numbering scheme for London became one of the many problems to be solved. Messrs. Laidlaw and Grinsted are usually given the credit for putting forward the first numbering scheme for an automatic system in London in a paper read before the Institute of Electrical Engineers in May, 1919. but a similar plan had been suggested by Mr. M. C. Pink of the London Telephone Service in an article in the Telegraph and Telephone Journal in June, 1915.
Pink proposed dividing London into nine divisions of 100,000 [lines] each, each division to be sub-divided into ten exchange areas with 4-digit numbers. As most of the manual exchanges in London already had 4-digit schemes the number of changes in subscribers numbers would have been small if the existing exchange areas were unchanged. In Pinks scheme the subscribers would have had to dial six digits, one for the division, one for the area plus four digits in the subscribers number. For example, if Putney were in the 6th area of division 2, in order to obtain Putney 2345 a subscriber on another exchange would dial 2-6-2345.
Laidlaw and Grinsteds scheme was similar but it was worked out in more detail and introduced the use of names for divisions. They proposed that the names should be engraved on the dials and so the number of digits to be remembered by subscribers would be limited to six. The names which they suggested for the divisions were: North, South, East, West, Kent, Victoria, Piccadilly, City and Bank. At the end of the first world war there were fourteen automatic exchanges in this country but all the London exchanges were still manual.
The Americans had a similar problem of converting New York to automatic working and they had decided to use the Panel system. This was a system using translators in which subscribers dialled the first three letters of the called exchange name followed by four numerical digits of the called subscribers number, the telephones being fitted with dials on which were engraved letters of the alphabet in order, in fact the type of dial which is used at present in director systems. The instructions to subscribers were simplified by printing the three initial letters of the exchange names in heavy type in the telephone directory. This system overcame most of the difficulties of Laidlaw and Grinsteds scheme and had the following advantages:
- 1. Advance preparation for transfer from manual to automatic working could be made at all exchanges.
- 2. Exchanges could be converted in any order.
- 3. No change in subscribers numbers was necessary although some exchange names would have to be changed.
- 4. Subscribers would have a uniform method of calling.
- 5. Only one alteration would be needed to the telephone directory.
- 6. Changeover arrangements were simplified.
The system was strongly commended by the American Bell Company; the Post Office administration was impressed by it and by the high reputation of the American service and American Telephone and Telegraph Company which had staked a large amount of capital on its success. The panel system was examined by the Post Office and although there had been experience of the working of only one exchange of this kind at Omaha, Nebraska, negotiations were started with the Western Electric Company for the installation of similar equipment in London and for the first exchange to be opened at Blackfriars.
When all negotiations had been completed, but before the signature had been put to the contract, the Automatic Telephone Company, Liverpool, produced an alternative scheme which combined the most attractive features of the panel system with the step-by-step system which was already in use at the majority of the British Automatic exchanges. The system was known as the Director system; it incorporated a translating facility and used the three letter and four figure numbers as in the panel system, it had additional advantages:
- 1. The cost was lower.
- 2. The circuit design was simpler.
- 3. British P.O. staff were already familiar with step-by-step equipment and the introduction of an additional type of equipment for maintenance was avoided.
- 4. It was suitable for small exchanges which the panel system was not
- 5. Introduction could be made quickly because British manufacturers were already tooled for the manufacture of step-by-step equipment.
- 6. Capital would be spent in this country instead of abroad.
It was decided because of these advantages to adopt the system for London. The equipment for the panel system was to have been manufactured at Antwerp, and after the decision to adopt a director working the Western Electric Company made a claim against the Post Office for the cost of tooling of their Antwerp factory.
Initially, two demonstration director exchanges were built. One was installed in G.P.O. Headquarters and the other was installed and exhibited at the British Empire Exhibition in 1924. Both of these exchanges were designed to work as two-letter plus four numerical or three-letter plus four numerical systems.
It was decided also to use the director system in Birmingham, Glasgow, Liverpool and Manchester, when the manual exchanges in those cities were changed to automatic working. Although the estimated growth of the provincial systems could have been contained in the two-letter plus four numerical system it was decided to adopt the three-letter plus four-figure system, as in London, as it would reduce the number of exchange names to be changed and would avoid difficulties for subscribers who were used in the London system. In addition, it was considered that three-letter codes were more significant and easier to remember than two-letter codes. Some exchange names had to be changed because 0 was used for calling the operator and there was then no Q on the dial. The number to be changed was small, Oldbury to Broadwell in Birmingham, Oldham and Openshaw in Manchester to Main and East, and Queens Park in Glasgow to Pollok. The director system was considered to be too expensive for systems smaller than London and the four large cities.
Non-director systems. At the same time as the introduction of director working was being discussed, a good deal of discussion and argument was going on about the numbering schemes for non-director systems. The Administration had strong views on the number of digits which a subscriber should be asked to remember. It was suggested that five digits would not be easily remembered by subscribers and that operators would make mistakes when passing forward numbers. The Administrations aim, therefore, was to reduce the number of digits in subscribers numbers to the smallest number. Similar arguments had been used many years before when the disadvantages of five-figure numbers had been recognised and car numbers were limited to .four numbers. Five digits were not, however, an innovation for they were in use in the Leeds system which had opened in 1918 and where the number of errors was no greater than in four-figure systems. In order to reduce the number of numerical digits to four or less several alternate numbering schemes for multi-exchange non-director systems were considered.
- 1. Numbers preceded by an initial letter of exchange name, using a dial with engraved letters, individual to each exchange.
- 2. Use of standard exchange names engraved on dials.
- 3. Introduction of standard exchange names using the director type dial.
- 4. The full exchange names engraved on dials for each system.
The suggestion of initial letter codes was abandoned because of the cost of special dials for each exchange, exchange names would have had to be changed if initial letters clashed and because of the problem of operators who had to dial into more than one automatic system. The use of standard exchange names with the director type dial, or with the names engraved on the dial, removed the objection of having to manufacture many different types of dial plate. Three sets of standard names for main and satellite exchanges were suggested:
a b c Central Central Central City Denmark Empire Royal Gordon Governor North Kensington Kingdom South Midland Monarch East Paddington Regent West Victoria Victoria
Those in (b) and (c) were chosen as being suitable for use with the director type dial.
It had already been decided to use 0 to call the operator, 9 for service purposes and not to use 1 because of the risk of false impulses. All the schemes, therefore, limited the number of exchange names to seven and had the disadvantage that in large systems a combination of exchanges would often be necessary with consequent changes in call charges, also they did not allow the flexibility which was required for a growing system. The schemes, which required exchange names to be engraved on dials, would be difficult to operate while some of the exchanges remained manual and both alternatives were unsatisfactory. It would be necessary either to replace dials at subscribers premises as exchanges were converted or subscribers would have to be given complicated instructions which would lead to confusion and the risk of misoperation. In addition, the procedure for operators would be made difficult at exchanges when they dialled into distant systems which were partly manual and partly automatic.
The Engineering Department and Traffic Section of Headquarters did not favour any of the alternative schemes and pressed for the use of five digit numbers. The Administration agreed reluctantly that five-digit numbers could be used if the personal agreement of the Secretary was obtained, but laid it down that the number of digits should be reduced to a minimum and with this aim in view mixed 3-and 4-, and, mixed 4- and 5-figure schemes would be permitted.
It was decided, however, to give a trial to the fourth of the alternatives, that is the use of dials with the exchange names engraved in full on the dial. This scheme was introduced at Brighton and was also considered but not adopted for Blackpool. The Brighton dial was a well-known oddity for years. In addition to be fitted on subscribers telephones it was fitted at switchboard positions at exchanges which dialled into the Brighton system. In practice it was found that five-digit numbers led to no great difficulty, and after a number of years the strong views held against such numbers were forgotten, and as the Brighton system had no great advantages it was finally abandoned in 1934.
Stage 4. Immediately after the second world war many exchanges were outgrowing their five-digit numbering schemes and conversion to director working, with 3-letter plus 4-digit or 2-letter plus 5 digits was considered, the first two to be examined being Edinburgh and Bristol. For Edinburgh it was decided to adopt director working with a 3-letter and 4-figure system, mainly on grounds of prestige, but although a decision was given to convert Bristol to director working, this was later reversed and it was decided to expand it to a six-digit numbering scheme. In fact Leeds, which was the first to have five-digit numbers, was also the first exchange to have six-digit numbers, these being brought into use in 1951. Six-digit numbering schemes have now been introduced in many other multi-exchange systems and no more director systems will be opened.
Stage 5. Little need be said in the present article about the introduction of the national numbering scheme in 1958, as this has been fully covered elsewhere. Suffice it to say here that the scheme, which was a pre-requisite for subscriber trunk dialling (S.T.D.) was designed to cater for national needs for the foreseeable future. Letters and figures were combined to form national dialling codes and, when the S.T.D, service was opened at an exchange, customers had to have a dial showing letters and figures.
Stage 6. The growth in the number of telephones in the last few years has been so great that sufficient acceptable exchange names cannot now be found for the additional exchanges which will be required shortly in the London director system and in other director systems in the future. Also, international dialling is growing so rapidly that the national numbering scheme must be designed to make easy the dialling of calls to this country from abroad; also, many countries do not have lettered dials and some do not use the roman alphabet. For these reasons the use of all-figure numbers was started on 14th March, 1966, ignoring all the wisdom of the pundits of the past on the maximum number of figures in a customers number and on the various mixtures of letters and figures.
I am grateful to Mr. R. B. Tupling for reading the draft of this paper and for his suggestions which have been included.