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In the administration of the North American Numbering Plan, central office code protection is a numbering policy for maintaining local seven-digit dialing in communities that extend on both sides of the boundary line between multiple numbering plan areas (NPAs), such as in cross-border towns on state lines.
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.
NXX often denoted NPA: Numbering plan area code The first NXX block is the numbering plan area code. When the second and third digits are the same, the code is an easily recognizable code (ERC). ERCs designate special services; e.g., 800 for toll-free service. The NANP is not assigning area codes with 9 as the second digit. [37]
Local number portability allows a number to be moved to a different carrier or a different wire center within the same rate center. [4] A landline provider typically will not allow a cross-town move to a different rate center under the same number at standard rates; possible alternatives include expensive foreign exchange service or a nomadic class of service such as voice over IP or a mobile ...
The Local Exchange Routing Guide (LERG) is a database of telecommunications numbering resources for use in the administration and operation of the North American Numbering Plan (NANP) by the NANP administrator (NANPA) and telecommunications service providers.
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 with new equipment. The first interchangeable NPA codes were area code 334 in Alabama and area code 360 in Washington state only, which entered mandatory use on January 15, 1995.
NPA-911 is forbidden as 9-1-1 is an emergency telephone number. (This is less restrictive than the rules prohibiting all three-digit N-1-1 codes as exchanges in all geographic area codes.) NPA-555 is reserved in every toll-free area code (except 800) for future information or directory assistance applications
Canadian (and other North American Numbering Plan) telephone numbers are usually written as (NPA) NXX-XXXX. For example, 250 555 0199, a fictional number, could be written as (250) 555-0199, 250-555-0199, 250-5550199, or 250/555-0199. The Government of Canada's Translation Bureau recommends using hyphens between groups; e.g. 250-555-0199. [2]