Telephone number pooling

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. In the United States it replaced the practice of allocating all 10,000 numbers of a central office prefix at a time. Under number pooling, the entire prefix is assigned to a rate center, to be shared among all providers delivering services in that rate center. Number pooling reduced the quantity of unused telephone numbers in markets which have been fragmented between multiple service providers, avoided central office prefix exhaustion in high growth areas, and extended the lifetime of the North American telephone numbering plan without structure changes of telephone numbers. Telephone number pooling was first tested for area code 847 in Illinois in June 1998, and became national policy in a series of Federal Communications Commission (FCC) orders from 2000 to 2003.

The North American Numbering Plan is a closed numbering plan, meaning that it assigns telephone numbers to individual endpoints based on a fixed-length telephone number. The national telephone number consists of a three-digit area code, a three-digit central office code, and a four-digit line number. Thus, each central office provides a resource of 10,000 telephone lines with a unique number each. While often enough for small communities, most cities require multiple central offices to service the community.

In the North American Numbering Plan, mobile telephones do not use distinct area codes from wireline services, but many central offices provide only wireless services, or just wireline services.

After the breakup of the Bell System on January 1, 1984, most telephone service areas in the United States were dominated by one carrier which held a monopoly on local service. The landline telephone systems evolved over a period of over one hundred years before diversification, so that it was technically difficult to share infrastructure between multiple providers. Central offices were established based on local demand and convention, and dialing prefixes were assigned to single providers who managed the plant in each location. This allocated all ten thousand line numbers of the central office code to one provider, even when demand could not exhaust the numbering pool. A new, competitive provider for the same area would be assigned the entire number space of a new central office prefix, reducing the overall efficiency of number utilization in the area.

Widespread introduction of the Advanced Mobile Phone System (AMPS) mobile phone service in 1983 created two competing carriers in each service area. Additional mobile carriers entered the market to provide digital service (such as GSM, introduced in 1991). In 1985, competitive access providers (CAPs) began to offer private line and special access services; originally based on private branch exchange (PBX) standards such as direct inward dial, these evolved into competitive local exchange carriers (CLEC). The Telecommunications Act of 1996 required incumbent telephone companies (ILECs) to interconnect with the new entrants.

Deployment of cable modems and voice over IP in the 1990s further blurred the boundaries between telcos and cable television providers. Every broadband Internet provider could become a telephone company, with telephony merely being one more application running over the packet-switched network. There was no requirement that the Internet to telephony gateway be operated by a facilities-based telco or cable company; anyone could buy a large block of telephone numbers from a CLEC, deploy a server to feed the calls to broadband Internet and offer telephone service.

With the advent of competition, each individual carrier required its own prefixes in each rate center, depleting available prefixes within high-growth and high-competition areas. Many of these new prefixes were sparsely used. This led to a rapid increase in the introduction of new area codes.

Public resistance to the introduction of new area codes, whether as overlay complexes (which allowed customers to keep their existing numbers, but broke seven-digit local calling) or by area code splits (where the area code of existing numbers was changed), prompted the FCC and state commissions to introduce thousands-block number pooling, i.e. the allocation of number space in blocks of only 1,000 numbers (area code and one digit of line number), rather than of an entire central office prefix with 10,000 lines.