020

Before the introduction of national dialling codes, the area now served by 020 had a large multiple telephone exchange system, called the London telephone area. The first exchange in this area, Central, was opened in the City of London on 1 March 1902.[3] The Director telephone system was developed so that subscribers in London could call one another in a linked numbering scheme regardless of whether they were on a manual local exchange or an automatic one.

In 1927, Holborn, the first Director automatic exchange in London, was cut over to the new system at midnight on Saturday 12 November. Because it was mainly a business exchange, most subscribers did not use the new system until Monday 14 November. The successful changeover was delayed by subscribers' unfamiliarity with dialling. Bishopgate and Sloane exchanges followed in six weeks, after which came Western and Monument exchanges. Because the London area contained 80 exchanges, complete conversion took many years.[4][5]

By 1934, the London telephone area comprised all 147 exchanges within 12.5 miles (20.1 km) of Oxford Circus.[6] By 1950, the London Director system had 75 exchanges within a 5-mile (8.0 km) radius of Oxford Circus and a further 65 in the 5–10-mile (8–16 km) belt. In Greater London (in other words, within 20 miles (32 km) of Oxford Circus), there were 237 exchanges.[7] In January 1956 a new director exchange – "SKYport" – was opened at London Heathrow Airport.[8]

From 1922, the first three digits of a seven-digit subscriber number – in other words, the local exchange codes – were represented with letters by way of a mnemonic. Each three-character code corresponded to an exchange within the London telephone area. The subscriber numbers were written, for example, as "ABBey 1234" and "WIMbledon 1234" or "ABB 1234" and "WIM 1234". By 1965, there were 350 local exchanges in London and the number of permutations that could be used for exchange names had been exhausted.[9] With the change to all-figure dialling in 1966, the system of mnemonics was withdrawn and the three-digit local exchange codes of many subscribers were altered. The old codes continued to work in parallel with new codes until January 1970, when the "ANN: All-figure Numbers Now" advertising campaign prompted callers to use only the new codes.[10] The transition to all-figure dialling allowed the codes to be grouped into eight sectors; all exchanges within 6 kilometres (3.7 mi) of the centre of London formed the Central sector and the other sectors radiated from it.[11]

The first three digits of a subscriber number continued to indicate the exchange to which the number belonged; for example, "222 1234" was in Westminster (Central Sector) and "946 1234" in Wimbledon (South West Sector).[notes 2] More than one local exchange code was usually overlaid for each area, so all Wimbledon numbers did not necessarily begin "946". Subscriber numbers changed to eight digits in 2000 when an additional 7 or 8 was added to each local exchange code (for example, "7222 1234" in Westminster and "8946 1234" in Wimbledon).

The STD code 01 was assigned to the London telephone area on 6 April 1959 as part of preparations for subscriber trunk dialling.[12] For the next thirty years, "01" became synonymous with the capital.[13] Until May 1990, the 01 code covered the same area as the current 020 code and had capacity for fewer than 10 million telephone numbers.[14]

In May 1990, the 01 code was abandoned and the area divided between 071 – which covered exchanges in the Central sector – and 081, which covered all other sectors. Exceptionally, two exchanges in the East sector[notes 3] covering the London Docklands redevelopment area were assigned the 071 code.[15] The anticipation that the code associated with central districts would be more prestigious than the other associated with the outer suburbs[16] was used as a plot device in the Essex-based TV comedy series Birds of a Feather.[17] At the time of the split, there were five local exchange codes assigned to Mercury Communications[notes 4] and numbers in these ranges could be assigned to either code.[15] This area code split doubled the potential capacity.[14] In 1995, on "PhONEday", the codes changed again, to 0171 and 0181.

Sequence of code changes.

The split into two area codes only lasted a decade. As part of the Big Number Change on 22 April 2000, the 0171 and 0181 codes were replaced with 020, following a period of dual-running that began on 1 June 1999 when the new 020 code was activated and ended on 14 October 2000 when the old 0171 and 0181 codes were finally ceased. The 22 April 2000 change also affected subscriber local numbers which gained an extra digit. "0171-xxx xxxx" numbers changed to "(020) 7xxx xxxx", while "0181-xxx xxxx" numbers became "(020) 8xxx xxxx". As a result of this history, there is now a widespread misconception that 0207 and 0208 are the dialling codes for parts of London.[18] This was exacerbated when local numbers beginning 3 started to be issued.

The reunification under a single code created capacity for approaching 100 million telephone numbers[14] and, starting in 2005, subscriber numbers beginning with the digit "3" were issued alongside those beginning "7" and "8".[14] In August 2019, Ofcom announced that subscriber numbers beginning with the digit "4" will also be introduced.[19]

The London telephone area operated as a single group for charging purposes in which all calls were priced at local rate. Additionally, as was usual, calls to and from the adjacent charge groups were charged as local.[notes 5]