A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a telephone network. A closed numbering plan, such as found in North America, imposes a fixed total length to numbers. An open numbering plan features variance in the length of telephone numbers.
A dial plan establishes the expected number and pattern of digits for a telephone number. This includes country codes, access codes, area codes and all combinations of digits dialed. For instance, the North American public switched telephone network (PSTN) uses a 10-digit dial plan that includes a 3-digit area code and a 7-digit telephone number. Most PBXs support variable-length dial plans that use 3 to 11 digits. Dial plans must comply with the telephone networks to which they connect.
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 and Canada began in the late 1940s 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 incoming 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 rural Texas (915). This numbering strategy became unnecessary when touch-tone phones arrived, as the tone allowed instant entry of digits.
The original set of North American area codes were unique. When the North American Numbering Plan was first developed, logic circuits consisted of relays and vacuum tubes, and simplicity was mandatory. The second digit of all area codes was 0 or 1, while the second digit of the exchange triplet was never 0 or 1, thus facilitating readily the recognition whether the dialing of a full 10-digit number was in progress or whether the user was dialing only a local number. In this coding scheme, a leading 1 indicating long distance, or out of the area code call, was not necessary. In some regions, local numbers were 7 digits (occasionally less), but a toll call within the area code required a preceding “1”. This told the subscriber that the call being placed was a toll call. Area Code 617 in eastern Massachusetts used this system in the early 1970s, while the Chicago area (Area Code 312) did not. In the Chicago Area, one could call the Boston area by dialing only 10 digits, while in Boston, to call Chicago, one would be required to use 11 digits (preceding the 10 digits with a “1”)
By the 1990s, the mechanical Central Office Switches were rapidly being replaced with ESS machines (Electronic Switching Systems), and the Area Code logic was no longer necessary. The need for more telephone numbers was increasing rapidly, and the NANPA (North American Numbering Plan Administration) was running out of “n0n” and “n1n” combinations. This Area Code scheme was abandoned, with the result that former Area Code only numbers could now be Exchanges, and former Exchange only numbers could now be Area Codes. This caused a logic dilemma: The Switch had no way of knowing whether to expect 10 digits or 7 digits. The solution was simple. If a preceding “1” was entered (by dial and Touch-Tone key pad), then 10 more digits were expected. If the first digit entered was NOT a “1” only 7 digits were expected and the call would remain within area code. For a short while, in some area codes, one could enter the full 11 digits for a call within their own neighborhood or just enter the last 7 digits, and the call would be routed and billed identically.
Currently, because of Area Code overlays, nearly all metropolitan and many rural telephone calls require the full 11 digit entry to complete a telephone call.
Apart from the use of numbering plans for telephone numbers, they are also used in routing of SS7 signalling messages as part of the Global Title. In public land mobile networks, the E.212 numbering plan is used for subscriber identities (e.g. stored in the GSM SIM) while E.214 is used for routing database queries across PSTN networks.
Country code - necessary only when dialing to phones in other countries. In international usage, telephone numbers are prepended 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 (also known as International Dialing Digit) 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, for example "011" in NANP countries (including Canada, Bermuda, and the United States): "011-XX-YYY-ZZZ-ZZZZ", or "00" in most European countries: "00-XX-YYY-ZZZ-ZZZZ". This can cause confusion as a different Access Code may be used 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.