In the United Kingdom and the Crown Dependencies, telephone numbers 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. Telephone numbers are of variable length. Local numbers are supported from landlines. Numbers can be dialled with a '0'-lead prefix that denotes either a geographical region or another service. Mobile phone numbers have distinct prefixes that are not geographic, and are portable between providers.
Structure All mobile numbers, nearly all geographic numbers, and nearly all non-geographic numbers have ten national (significant) digits after the trunk code 0. 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, the "area code" was historically referred to as a "subscriber trunk dialling code" (STD code) or a "dialling code". 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 part has more digits. Local customer numbers are four to eight digits long. The total number of digits is ten or, very rarely, nine. 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, these countries are closely related.
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 The 0500 range had NSN length as 9 digits only, and was withdrawn from use on 3 June 2017.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 nine or ten digits after the 0 trunk code or +44 international dialling prefix.
Four-digit area codes Four-digit area codes have either six-digit subscriber numbers or a mix of five- and six-digit subscriber numbers.
(01xxx) xxx xxx This is the format used by most areas. It has a four-digit area code (after the initial zero) and a six digit subscriber number, and is known as 4+6 format. These area codes were changed by adding a "1" directly after the initial zero as a part of PhONEday in 1995. Just short of 581 areas use this format, and the area codes range from 01200 to 01998. Almost all (01xxx) area codes now have only six-digit subscriber numbers, but a small number of these areas also have some subscriber numbers that are only five digits in length (see next section).
Six of the four-digit area codes are known as "mixed" areas as they share those four digits with the twelve five-digit area codes. This leads to a restriction as to which initial digits can be used for subscriber numbers within those four-digit area codes, e.g. in the 01387 four-digit area code, subscriber numbers cannot begin with a 3 because 013873 is a separate five-digit area code; likewise in the 01946 four-digit area code, subscriber numbers cannot begin with a 7 because 019467 is a separate five-digit area code.
(01xxx) xx xxx This is used for forty smaller towns which have a mixture of six and five digit local numbers, each type allocated in specific DE blocks*; e.g. in the 01647 area code numbers beginning 24 and 61 have five digits (24xxx and 61xxx; known as 4+5 format) whereas all other DE blocks* within that area code have six digit numbers. The number of places with five digit subscriber numbers and an 01xxx area code has declined rapidly in recent decades. There were 511 ranges allocated across 56 different area codes in January 1998. The Big Number Change removed many, especially in Northern Ireland, and by July 2005 there were only 329 ranges in 42 codes. By April 2010 this had reduced to 324 ranges in 40 codes, with still the same number in November 2012. The 40 area codes are listed in the table below. *A DE block is a block of numbers where (taking the area code and the subscriber number together) the initial 0 and the next six digits after it are the same for all the subscriber numbers in the block. (These area codes, like many others, were changed by adding a "1" directly after the initial zero as a part of PhONEday in 1995.)
Three-digit area codes Three-digit area codes are generally used for the UK's largest cities and conurbations (with the exception of London) and always have seven-digit subscriber numbers and always begin 011x or 01x1.
(01x1) xxx xxxx This is the geographic number format for the first round of five large cities moved to all figure dialling in the 1960s, and subsequently also used in the metropolitan county of Tyne and Wear, eastern County Durham and south-eastern Northumberland from the 1980s onwards. These six areas have a three-digit area code matching the pattern 1x1 (after the initial zero) and a seven digit subscriber number, and this is known as 3+7 format. These area codes were changed by adding a "1" directly after the initial zero as a part of PhONEday in 1995.
(011x) xxx xxxx This is the geographic number format for the second round of large cities and towns moved to brand-new three-digit area codes. Five of these were moved in 1995 as a part of PhONEday, with Reading then following a year later. At the time of the change, an extra digit was added to the subscriber number. These six areas have a three-digit area code matching the pattern 11x, with a seven-digit subscriber number, and this is known as 3+7 format. The first three digits of the local number identifies a small area within the town or city. The former Reading area code had already been changed once, by adding a "1" directly after the initial zero as a part of PhONEday in 1995.
Two-digit area codes Two-digit area codes always have eight-digit subscriber numbers and always begin 02.
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