Telephone Area codes

Area codes are also known as Numbering Plan Areas (NPAs) and formerly known as STD codes in the UK. These are necessary (for the most part) only when dialed from outside the code area, from mobile phones, and (especially within North America) from within overlay plans. Area codes usually indicate geographical areas within one country that are covered by perhaps hundreds of telephone exchanges, although the correlation to geographical area is becoming obsolete.

The area code is usually preceded in the dial string by either the national access code ("0" for many countries, "1" in USA and Canada) or the international access code and country code. However, this is not always the case, especially when 10-digit dialing is used. For example, in Montreal, Quebec, where area codes 514, 438, and 450 are currently in use, users dial 10-digit number (ex 514 555 1234), dialing a 1 before this results in a recording advising not to dial a 1 as it is a local call. For non-geographic numbers, as well as mobile telephones outside of the North American Numbering Plan area, the "area code" does not correlate to a particular geographic area.

Area codes are often quoted including the national access code, for example a number in London: 020 8765 4321. Users must then correctly interpret the "020" as the code for London. If calling from another number in London they merely dial 8765 4321, or if dialing from another country, drop the "0" and dial: +44 20 8765 4321.