<P> 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 . For public number systems, geographic location plays a role in the sequence of numbers assigned to each telephone subscriber . </P> <P> 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 . Many numbering plans 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 . </P> <P> 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 . </P> <P> 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 controls internal communications between telephone extensions . </P>

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