Callers within the geographical area of a given area code usually do not need to include this particular area code in the number dialed, thereby giving the caller shorter local telephone numbers. In international phone numbers, the area code directly follows the country calling code.
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2 United Kingdom
4 See Also
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United States and Canada
In the United States and Canada, area codes are regulated by the North American Numbering Plan. Currently, all area codes in the NANP must have 3 digits. Many other countries have area codes that are shorter for heavily populated areas and longer for lightly populated areas. Area codes are also referred to as NPAs, for Numbering Plan Area.
Before 1995, North American area codes were of the form [2-9][0/1][0-9], with the prefix or NNX in the form [2-9][2-9][0-9]; that codespace filled up due to overallocation of , and was extended to [2-9][0-8][0-9]-[2-9][0-9][0-9] (referred to as NPA-NXX).
Not all area codes correspond to geographical area. Codes 8xx with the last two digits matching, such as 800, 888, 877, 866, etc. are reserved for toll free calls. Code 900 is reserved for premium-rate calls (also known as dial-it services).
There are a number of proposals for what the NANP should do when this larger space fills up. In one proposal, existing codes may be changed to "a9bc" (e.g. San Francisco 415 would become 4915); once that conversion is complete, the new second digit would be opened for a new range. Other proposals include reallocating blocks of numbers assigned to smaller long distance carriers or unused reserved services.
None(?) of these changes enable the existence of variable length area codes, which are commonplace outside the USA (See United Kingdom below). Also see 
There are two noteworthy peculiarities in the US numbering:
A short list of examples, set out in the officially approved (Oftel) number groups:
Area codes were first introduced in 1958, allowing a caller to call another telephone direct instead of via a manual telephone exchange, a process known as Subscriber Trunk Dialling (STD), although the process was not completed until 1979. The four-digit codes were originally assigned based on the first three letters of the respective place's name and the corresponding numbers on a telephone keypad. For example Aylesbury was given the STD code (0)296, where the letter (a) can be found on the number 2, the letter (y) on the number 9 and the letter (l) on the number 6. However as more and more places were given STD codes this system became unworkable.
Five years later in 1995, the whole country was running out of numbers, so nearly all local codes were given an extra initial 1. Thus, central London's (0)71 became (0)171.
Amid criticism for not having made adequate provision in 1995, on April 22, 2000, numbers in London Portsmouth Southampton Cardiff Coventry and Northern Ireland changed again to give an eight digit local number and a three digit area code - (0)20 in the case of London, for example. This was part of a larger restructuring of all UK codes.
In the Netherlands, the area codes are -- excluding the leading '0' -- one, two or three digits long, with larger towns and cities having shorter area codes permitting a larger number of telephone numbers in the ten digits used.