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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 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. This made central office codes interchangeable. An interchangeable NPA code and an interchangeable central office code is a code that permits any of the ten numerals in the middle ...
Central office code protection was once common in communities on provincial or state boundary lines. It has been declining in use as inefficient allocation of numbering resources to the growing number of competitive local exchange carriers has caused depletion of available number prefixes, often requiring ten-digit local calls and overlay plans ...
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.
Area code 564, originally planned for introduction in October 2001, was delayed indefinitely in August 2001 after state regulators determined that the use of number pooling had resulted in more efficiency in the use of telephone numbers in western Washington. The 564 overlay complex was delayed until 2017. Number pooling is not practiced in Canada.
Conservation measures, such as number pooling, mitigated the immediate need for relief. [5] More than twenty years after that announcement, area code 557 was overlaid on August 12, 2022, as the available numbers in 314 neared exhaustion. [6] Prior to October 2021, area code 314 had telephone numbers assigned for the central office code 988.
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Under all-number calling, the number of permissible central office prefixes increased from 540 to potentially 800, but the first two digits of the central office code were still restricted to the range 2 to 9, and the eight combinations that ended in 11 were reserved as special calling codes. [2]