Open dialling plan

A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of administrative regions of the public switched telephone network (PSTN) and they are also present in private telephone networks.

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, distinguishing open numbering plans and closed numbering plans. A closed numbering plan imposes a fixed number of digits assigned to every telephone, while an open numbering plan features a variable length of telephone numbers assigned to stations. An open plan permits the expansion of the total numbering capacity of the plan by addition of more digits to a subset of numbers. Many numbering plans subdivide their territory of service into geographic regions designated by a special prefix, often called area code, which is a set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber. For example, the North American Numbering Plan, which is a closed numbering plan, prescribes ten digits for each complete destination routing code, including a three-digit area code. Other countries with open numbering plans use variable-length numbers; in some, such as Finland, subscriber numbers may vary in length even within an exchange.

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, however, imposing a maximum length of 15 digits to telephone numbers. The standard defines a country calling code (country code) for each state or region which is prefixed to each national numbering plan telephone number for international destination routing.

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