Number pooling

Number pooling is a method of reallocating telephony numbering space in the North American Numbering Plan, primarily in growth areas in the United States.

Instead of allocating blocks of ten thousand numbers to each carrier in each community, a block of ten thousand numbers is assigned to an individual geographic rate center. That block is then split into ten blocks of a thousand numbers each, which can be separately assigned to competitive local exchange carriers by a number pooling administrator. This reduces the quantity of wasted numbers in markets which have been fragmented between multiple carriers.

The North American Numbering Plan is based on fixed-length telephone numbers; when area codes (1947) and direct distance dialling (1951) were first introduced, North American numbers were gradually extended to a fixed length in the format 1 + three-digit area code + three-digit exchange prefix + four-digit subscriber number. Each central office code (exchange prefix) contained 10,000 possible local numbers - enough for a village or small town telephone exchange. A mid-size city would have multiple exchanges with multiple CO codes assigned to each.

North American mobile telephones, from their introduction in 1983, have used local numbers from the same geographic area codes as wireline services. Unlike the system in nations where mobiles have their own special area code, the recipient of a mobile call in the US or Canada pays the airtime charges.

In most areas, one carrier held a monopoly on local service. The landline telephone system was originally constructed in an era before transistorisation, remote concentrators or cabinetisation. Individual lines from subscribers ran not to a remote concentrator but directly to the nearest village community dial office, to be terminated at an automated mechanical switch or manual exchange. To keep subscriber loop wiring requirements manageable, local exchanges were built in each tiny hamlet. Rural subscribers often made do with party line service. Each village was a separate rate center for long-distance calls and each had its own block of ten thousand numbers (the largest that reasonably could fit on a small-town manual cord switchboard with a jack and indicator for each line). In most small villages, many of these numbers would remain unused. In large cities, multiple exchanges were created to cover individual districts or neighbourhoods; each had its own series of distinctive prefixes, and multiple blocks of ten thousand numbers each. The first few digits of a number uniquely identified a village, a town or a neighbourhood of a big city.

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