A short history of 0203 phone numbers in London
020 is the area code used for phone numbers in the London area. The original STD (Subscriber Trunk Dialling) area code for London was 01 when the numbering plan was first made available to the public in 1979. This in turn changed to 071 and 081 in 1990, and 0171 and 0181 in 1995.
The 020 area prefix was first created in June 1999 when the increase in demand for telephone numbers in the capital meant more number space was required. As a result of this, all 0171 and 0181 London numbers changed to 020 7XXX XXXX and 020 8XXX XXXX numbers.
Initially there was a common misconception that (020) 7 specifically related to inner London and (020) 8 was specific to greater London, however this isn't the case.
In June 2005, again to meet the growing clamour for London-based phone numbers, new London-centric numbers which followed the format of 020 3XXX XXXX were introduced.