The code is used at 170 telephone exchanges in and around Greater London as part of the largest linked numbering scheme in the United Kingdom.

In common with all other British area codes the initial '0' is a trunk prefix that is not required when dialling London from abroad.

020 is also one of the dialling codes used for telephone services on the remote South Atlantic island of Tristan da Cunha.

[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.

[9] With the change to all-figure dialling in 1966, the system of mnemonics was withdrawn and the three-digit local exchange codes of many subscribers were altered.

[notes 2] More than one local exchange code was usually overlaid for each area, so all Wimbledon numbers did not necessarily begin "946".

The STD code 01 was assigned to the London telephone area on 6 April 1959 as part of preparations for subscriber trunk dialling.

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.

[19] The London telephone area operated as a single group for charging purposes in which all calls were priced at local rate.

[notes 5] The code serves a roughly circular area with a radius of 20 kilometres (12 mi) from the centre of London.

[notes 6] Communities outside Greater London that are within the code are Buckhurst Hill, Chigwell, Loughton and Sewardstone in Essex; Borehamwood, Bushey, Carpenders Park, Elstree and South Oxhey in Hertfordshire; and Ewell, Molesey, Thames Ditton and Whyteleafe in Surrey.