020 is the national dialling code for London in the United Kingdom. All subscriber numbers within the area code consist of eight digits and it has capacity for approaching 100 million telephone numbers. 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. The 020 area code fully replaced older area codes for London on 22 April 2000, following multiple telephone number changes during the 1990s. As is the case for other codes in the UK, the 020 area code may also be used for services without any physical presence in the area, such as private networks or virtual numbering. As one such user is the Foreign and Commonwealth Office Telecommunications Network, provided by Global Crossing, 020 is also one of the dialling codes used for telephone services on the remote South Atlantic island of Tristan da Cunha. History London telephone area 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. 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. By 1934, the London telephone area comprised all 147 exchanges within of Oxford Circus. By 1950, the London Director system had 75 exchanges within a radius of Oxford Circus and a further 65 in the 5–10-mile (8–16 km) belt. In Greater London (in other words, within of Oxford Circus), there were 237 exchanges. In January 1956 a new director exchange – "SKYport" –