What is the definition of an area code?

An area code is a telephone numbering prefix used to identify a region of a country or a particular telephone service.

In North America (USA, Canada, etc.), the area code is always 3 digits, always followed by a 7-digit local number. For example, in (999) 555-0123, '999' is the area code.

Other countries have different systems. In many countries, the length of the area code varies according to the size of the main city in that area code. For example, in the UK, 020 is the area code for London, but 016973 is the area code for Wigton, a small village in Cumbria. Some countries don't have area codes at all: Denmark, Norway, Costa Rica, Vanuatu, and quite a few others.

In most countries, the area code is quoted including the trunk prefix 0. In most cases, you do not dial the domestic trunk prefix on international calls, so London's 020 area code becomes +44 20. However, there are a few exceptions, most notably Italy: Rome's 06 becomes +39 06, for example.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)