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Telephone number pooling, thousands-block number pooling, or just number pooling, is a method of allocating telephony numbering space of the North American Numbering Plan in the United States. The method allocates telephone numbers in blocks of 1,000 consecutive numbers of a given central office code to telephony service providers.
The Bell System engineers expected that the first numbering plan areas to require more than the 640 possible central office codes would not occur before c. 1973. [3] The solution for expanding the numbering pool within an NPA was to remove the restriction posed by not using 0 and 1 in the middle position of central office codes.
Thus, NXX is a number from 200 through 999, while XXXX is a range from 0000 through 9999. The first three digits of a telephone number are the numbering plan area code (NPA code, or simply NPA). The next three, NXX, identify the central office and the last four digits are the line number of an individual office.
For example, 234 235-5678 is a valid telephone number; with area code 234, central office prefix (exchange) 235, and line number 5678. The number 234 911-5678 is invalid, because the central office code must not be in the form N11. 314 159-2653 is invalid, because the office code must not begin with 1.
After World War II, the newly conceived nationwide numbering plan of 1947 sought to unify all local numbering plans by using a system of two central office letters and one digit to complete the office prefix, and four digits for line number. This system was referred to as 2L-5N, or simply 2-5. This plan was projected to be usable beyond the ...
For example, under this system, a well-known number in New York City was listed as PEnnsylvania 6-5000. In small towns with a single central office, local calls typically required dialing only four digits at most. A toll call required the assistance of an operator, who asked for the name of the town and the station number.
For most of the second half of the 20th century, it was possible to dial any number in the metro area with just seven digits. The entire area implemented a system of central office code protection in which no central office code was duplicated in the entire area. Each existing central office code was properly routed with each area code in the ...
The problem was exacerbated by Canada's inefficient system of number allocation. Unlike the United States, Canada does not use number pooling as a relief measure. Every competing carrier is assigned blocks of 10,000 telephone numbers, which correspond to a single central office prefix, in every rate centre in which it plans to offer service.