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Such networks operate a private switching system or a private branch exchange (PBX) within the network. The internal numbers assigned are often called extension numbers, as the internal numbering plan extends an official, published main access number for the entire network. A caller from within the network only dials the extension number ...
The preferred arrangement, over timing, was the prefix method, in which the subscriber had to start dialing ten-digit calls with a 0 or 1 preceding the full telephone number. The preference was based on Bell System behavioral research, but technical aspect also favored this arrangement in some cases, in that it could shorten post-dial delay ...
In NANP telephone number specifications, the letter N represents a numeral from 2 through 9, while the letter X represents any numeral. 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).
In areas in which overlays were used, this generally avoids the need for converting telephone numbers, so existing directories, business records, letterheads, business cards, advertising, and "speed-dialing" settings can retain the same telephone numbers, while the overlay is used for new number allocations.
In the early 1980s, Bell Labs received a patent for what became AT&T's "Advanced 800 Service", a computer-controlled system where any toll-free number could point to any destination number, such as to a small business local number instead of a special InWATS line, and an itemized bill generated only for the calls the business actually received.
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 ...
The prefixes in the Americas start with one of 1,2,5. All countries in the Americas use codes that start with "5", with the exception of the countries of the North American Numbering Plan, such as Canada and the United States, which use country code 1, and Greenland and Aruba with country codes starting with the digit "2", which mostly is used by countries in Africa.
A telephone prefix is the first set of digits after the country, and area codes of a telephone number. In the North American Numbering Plan countries (country code 1), it is the first three digits of a seven-digit local phone number, the second three digits of the 3-3-4 scheme.