Telephone numbers in the United Kingdom are administered by the Office of Communications (Ofcom). For this purpose, Ofcom established a telephone numbering plan, known as the National Telephone Numbering Plan, which is the system for assigning telephone numbers to subscriber stations. The numbers are of variable length. Local numbers are supported from land-lines or numbers can be dialled with a '0'-lead prefix that denotes either a geographical region or another service. Mobile phone numbers have their own prefixes which are not geographical and are completely portable between providers. Structure All mobile numbers, nearly all geographic numbers and nearly all non-geographic numbers have 10 national (significant) numbers after the "0" trunk code. The overall structure of the UK's National Numbering Plan is: A short sample of geographic numbers, set out in the officially approved (Ofcom) number groups: In the United Kingdom, area codes are two, three, four or, exceptionally, five digits long (after the initial zero). Regions with shorter area codes, typically large cities, permit the allocation of more telephone numbers as the local number portion has more digits. Local customer numbers are four to eight digits long. The total number of digits is ten or, very rarely, nine (not including the initial zero). The "area code" is also referred to as an "STD (code)" or a "dialling code" in the UK. The code allocated to the largest population is (020) for London. The code allocated to the largest area is (028) for all of Northern Ireland. The UK Numbering Plan also applies to three British Crown dependencies – Guernsey, Jersey, and the Isle of Man – even though they are not part of the UK. Format Possible number formats for UK telephone numbers are: Number ranges starting 01 can have National Significant Number (NSN) length of 10 or (very rarely) 9 digits. NSN is the number of digits after the leading 0 trunk code or +44 international prefix. The 0800 range can have NSN length as 10, 9 or 7 digits. The 0845 range can have NSN length as 10 or 7 digits. The 0500 range had NSN length as 9 digits only, and was withdrawn from use on 3 June 2017. All other UK numbers have NSN length of 10 digits. There are no telephone numbers in the UK with an NSN length of 8 digits. Geographic numbers Standard geographic numbers Geographic telephone numbers in the UK always have