A telephone numbering plan is a system of allocating and routing telephone numbers in a telephone network. A closed numbering plan, such as found in North America, features fixed length area codes and local numbers. An open numbering plan features variance in length of area code or local number, or both. The term dial plan should not be confused with numbering plan.

Structure

In early telephone systems, connections were made in the central office by telephone operators using patch cords to connect one party to another. If a person wanted to make a phone call, he or she would pick up a phone and wind a crank on the side. The crank was a small generator that would light a lamp at the central office. An operator would see the light and insert their patch cord into a socket and assist the customer with the call connection. The operator would use patch cords to connect the caller to the person being called. If the party being called was in another exchange, the operator would use a patch cord to connect to another exchange where an operator elsewhere would finish the connection. As technology advanced, electro-mechanical switches were introduced and calls were made using "rotary dials".

Initial use of area codes in the United States began in the 1950's with large cities. By 1966, the system was nationwide.

Area codes were assigned based on the length of time a rotary dial phone took to dial the area code. Densely populated areas like New York City, Chicago, Los Angeles, and Detroit had huge call volume and were assigned numbers (212, 312, 213, 313) that could be quickly dialed from a rotary dial phone. On a rotary dial phone low digits (1, 2, 3, 4) could dial quickly as the time the rotary dial took to return to the home position was minimal. High digit numbers (7, 8, 9, 0) on rotary dial phones took much longer to return to the home position and were usually used in less densely populated areas like Vermont (802), rural Texas (915), Tennessee (901), and the Canadian Maritimes (902). This system became unnecessary when touch-tone phones arrived, as the tone allowed instant entry of digits.

Country code - necessary only when dialing to phones in other countries. In international usage, telephone numbers should always be quoted with the country code preceded by a "+", and with spaces in place of hyphens (e.g., "+XX YYY ZZZ ZZZZ"). This allows the reader to choose which Access Code they need to dial from their location. However, it is often quoted together with the international access code which must precede it in the dial string, especially in the United States and Canada (e.g., "011-XX-YYY-ZZZ-ZZZZ"). This can cause confusion as "011" may not be a valid Access Code where the reader is located. (On GSM networks, "+" is an actual character that may be used internally as the international access code, rather than simply being a convention.)

Area code

Area code - 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. It must usually be preceded in the dial string by either the national access code or the international access code and country code. For non-geographical numbers, as well as mobile telephones outside of the United States and Canada, the "area code" does not correlate to a particular geographic area.