Editing Telephone numbering plan (section)

{{Short description|Numbering method for assigning routing addresses for telephones}}
{{pp-vandalism|small=yes}}
A '''telephone numbering plan''' is a type of [[numbering scheme]] used in [[telecommunication]] to assign [[telephone number]]s to subscriber telephones or other telephony endpoints.<ref>{{cite journal |last=Nunn |first=W.H. |title=Nationwide Numbering Plan |journal=Bell System Technical Journal |volume=31 |number=5 |page=851 |year=1952 |doi=10.1002/j.1538-7305.1952.tb01412.x }}</ref> Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined world-wide, as well as within each of the administrative regions of the [[public switched telephone network]] (PSTN), and in private telephone networks.

In public numbering systems, geographic location typically plays a role in the sequence of numbers assigned to each telephone subscriber. Many numbering plan administrators subdivide their territory of service into geographic regions designated by a prefix, often called an '''area code''' or '''city code''', which is a set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber. Within such regions designated by area codes, locally unique telephone number are assigned based on locally determined principles, but in agreement with the larger-network rules.

Numbering plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements. A broad division is commonly recognized between closed and open numbering plans. A '''closed numbering plan''', as found in North America, features fixed-length area codes and local numbers, while an '''open numbering plan''' has a variance in the length of the area code, local number, or both of a telephone number assigned to a subscriber line. The latter type developed predominantly in Europe.

The [[International Telecommunication Union]] (ITU) has established a comprehensive numbering plan, designated [[E.164]], for uniform interoperability of the networks of its member state or regional administrations. It is an open numbering plan but imposes a maximum length of 15 digits to telephone numbers. The standard defines a [[country code]] for each member region which is prefixed to each national telephone number for international [[destination routing]].

Private numbering plans exist in telephone networks that are privately operated in an enterprise or organizational campus. Such systems may be supported by a private branch exchange (PBX), which provides a central access point to the PSTN and also controls internal calls between telephone extensions.

In contrast to numbering plans, which determine telephone numbers assigned to subscriber stations, [[dialing plan]]s establish the customer dialing procedures, i.e., the sequence of digits or symbols to be dialed to reach a destination. It is the manner in which the numbering plan is used. Even in closed numbering plans, it is not always necessary to dial all digits of a number. For example, an area code may often be omitted when the destination is in the same area as the calling station.