Number pooling is a method of reallocating telephony numbering space in the North American Numbering Plan, primarily in growth areas in the United States.
Originally, in North America, individual telephone exchanges were assigned entire individual prefixes, with all the 10,000 possible numbers (0000 to 9999) having that prefix being available (and only available) to that exchange. ("Prefix", "NXX", and "exchange" are synonymous terms in NANP telephony.) Typically, one exchange served one municipality (or rarely, groups of closely associated municipalities). As the growth of an area led to increased demands for phone numbers, more prefixes would be added.
Along with the advent of competition among telephone carriers, as well as mobile telephone providers, each individual carrier serving a given municipality required its own prefixes. This began to put pressure on the prefixes available within high-growth and high-competition areas, and led to a rapid increase in the introduction of new area codes.
By the early 1990s, the NANPA was forced to change the format rules to increase the number of valid area codes. Previously, all area codes had 0 or 1 as their second (middle) digit; the rule change allowed any digit except 9 as the second digit.
However, public resistance to the introduction of new area codes, even overlay plans which allowed customers to keep their existing numbers (as opposed to split plans where the area code of existing numbers changes), prompted the FCC and state telecommunications commissions to introduce and encourage the allocation of number space in smaller blocks of 1,000 numbers, with each block consisting of a prefix and the first digit after the prefix. Local exchange routing databases now include a "block ID" to indicate the ownership of the specific sub-blocks within a prefix.