History of telephone numbers in the United Kingdom

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.