Telephone numbers in the United Kingdom have a flexible structure that reflects their historical demands, starting from many independent companies through a nationalised near-monopoly, to a system that supports many different services, including cellular phones, which were not envisaged when the system was first built. Numbers evolved in a piecemeal fashion, with numbers initially allocated on an exchange-by-exchange basis for calls connected by manual operators. Subscriber numbers reflected demand in each area, with single digit telephone numbers in very rural areas and longer numbers in cities. Beginning with London's director system, a need to automate telephone dialling resulted in every exchange being allocated either a unique Subscriber Trunk Dialling (STD) code or unique range of numbers within a wider dialling code area. For many years, calls dialled between nearby exchanges often required 'local codes' to select the most direct call route, rather than dialling the STD codes. Demand for telephone lines has grown and exchanges have been modernised, so many subscriber numbers have been lengthened and highly localised STD codes have been rationalised into wider area codes. Such was the demand for lines in London that the city's telephone area was first split into two separate dialling codes, before being merged again with a short dialling code and long subscriber numbers. Meanwhile, a need to find numbers for special services, such as mobile telephones and information services, initially led to confusion with traditional telephone numbers. From 1995, extensive renumbering exercises have led to specific number ranges being allocated to distinguish between traditional 'geographic' telephone numbers, mobile numbers and special services. Despite these rationalisations, there remains no standard format or length for a UK area code or telephone number, and there are misunderstandings in code areas which have seen alterations to customers' individual telephone numbers.
Origins The telephone service in the United Kingdom was originally provided by private companies and local councils. But by 1912–13 all except the telephone service of Kingston upon Hull and Guernsey had been bought out by the Post Office. The Post Office also operated telephone services in Jersey until 1923 and the Isle of Man until 1969 when the islands took over responsibility for their own postal and telephone services – although the Isle of Man system remained part of British Telecom until 1987. Post Office Telecommunications was reorganised in 1980–81 as British Telecommunications (British Telecom, or BT), and was the first major nationalised industry to be privatised by the Conservative government. The Hull Telephone Department was itself reconstituted as Kingston Communications, in 1987; it was sold by Hull City Council in the late 1990s and celebrated its centenary in 2004.
Subscriber numbers From the introduction of the telephone in the late 1870s, to the early 1990s, telephone numbers in most of the United Kingdom were usually shown with a written exchange name followed by the subscriber number, e.g. 'Mallaig 10' or 'Aberdeen 43342'. This allowed calls to be placed initially through the operator and later by using local or national dialling codes. Exchange names were usually closely tied to the physical location of telephone exchanges, being either the name of a city, town or village or district. The length of early telephone numbers depended on the number of subscribers attached to a particular exchange: if there were fewer than 10 subscribers, a single digit sufficed. One single-digit number survived in Rèinigeadal, Scotland until 1990. Two-digit numbers were sufficient on small exchanges with fewer than 100 subscribers, while exchanges with a few hundred lines used three-digit subscriber numbers, e.g. 200–499. Larger exchanges which covered thousands of lines used four-digit subscriber numbers, e.g. 2000–5999, or five-digit subscriber numbers, e.g. 20000–49999. As demand for telephones grew, more telephone numbers were required. This would often see an exchange with existing three-digit numbers open one or more new ranges with four-digit local numbers (e.g. 5000-6999), and exchanges with existing four-digit numbers open one or more new ranges with five-digit local numbers (e.g. 60000–69999). This piecemeal approach lasted until the 1980s when a shortage of numbers resulted in an overhaul of dialling codes and subscriber numbers. As of 2020, 52 exchanges still have five-digit subscriber numbers of which a dozen places also have unusually long STD codes. One area with a long STD code retains four-digit local numbering (see Telephone Numbers in the United Kingdom for examples).
Director system
In November 1922 the General Post Office decided to adopt the Strowger system after trialling several automatic exchange systems; initially for small and medium towns and cities in Britain. But for London this required development of the Director telephone system by the Automatic Telephone Manufacturing Company (ATM) to enable operation with a mixture of both automatic and manual local exchanges in the London area for several decades. A demonstration model of a "Director" exchange was shown by the developer ATM of Liverpool as part of the Post Office exhibits at the British Empire Exhibition at Wembley in 1924 and 1925. However, it was not until 1927 that the first "Director" telephone exchange was brought into service in Holborn, London and rolled out progressively across Greater London. Letters were assigned to each number from 2 to 0 on the telephone dial—in a different scheme than that currently used—and a three-digit code, represented by letters, identified the local exchange. Telephone numbers were displayed preceded by the exchange name, with the first three letters highlighted to indicate the code, and number, such as WHItehall 1212. Director schemes were gradually introduced in other major cities, Birmingham, Edinburgh, Glasgow, Liverpool and Manchester.
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