There is no such thing as a London 0207 number
Londoners often write their numbers as 0207 or 0208, but the area code is simply 020 and the local part has eight digits. The mistake is a fossil of two great renumberings. Britain's phone plan is a patchwork of code lengths, each layer recording when a town ran short of numbers.
Almost every UK number carries ten digits after the leading zero, but they split between area code and local number in several ways. Big cities get short codes so there is room for more subscribers: two digits with an eight-digit local number, three digits with seven, and so on. Most of the country uses the common pattern of a four-digit code and six-digit local numbers, found in just short of 581 areas. London's 020 serves the largest population, while 028 covers all of Northern Ireland, the biggest area. Guernsey, Jersey and the Isle of Man share the plan despite not being part of the UK.
Two events reshaped it. On PhONEday in 1995, nearly every geographic code gained a 1 straight after the zero, which is why most still begin 01. Then the Big Number Change in 2000 moved London, Cardiff and Northern Ireland, among others, onto new codes starting 02, lengthening local numbers so that London's gained one digit. Hence the confusion: people kept the old habit of reading the first digit of the local number as part of the code, producing the phantom 0207 and the equally wrong 02920 for Cardiff.
Some odd corners survive. A dozen small places use five-digit codes, which must share their first four digits with a parent code, so that in the 01387 area no local number may start with 3. Brampton is the last place in Britain with four-digit local numbers, and the count of five-digit local ranges has kept shrinking. Where numbers run short, the regulator Ofcom can force callers to dial the area code even for local calls, which frees up combinations that would otherwise clash with emergency numbers such as 999; Bournemouth's 01202 made that switch on 1 November 2012.
Mobiles sit under 07 and no longer reveal your operator because numbers are portable. Numbers starting 03, introduced in 2007, bill like ordinary local calls, unlike older 084 and 087 lines.
Source: Telephone numbers in the United Kingdom
