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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 ...
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.
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 ...
Area code 327 was assigned for this purpose in January 2010. [1] The release of little-used central office codes and the nationwide introduction of number pooling delayed the exhaustion of the numbering pool and the relief action was indefinitely suspended in June 2012.
This increased the numbering pool for central office codes to 640, and resulted in the partitioning of the prefix space (000—999) according to the table at the right. [ 5 ] All-number calling was first field-tested in Wichita Falls, Texas starting on January 19, 1958, with the installation of a new dial exchange. [ 3 ]
Download QR code; Print/export ... Central office code protection; E. Enhanced 911; L. List of area code overlays; M. ... Telephone number pooling;