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A numbering plan area with multiple area codes is called an overlay. Area codes are also assigned for non-geographic purposes. The rules for numbering NPAs do not permit the digits 0 and 1 in the leading position. [1] Area codes with two identical trailing digits are easily recognizable codes (ERC).
Toll-free telephone numbers in the North American Numbering Plan have the area code prefix 800, 833, 844, 855, 866, 877, or 888. Additionally, area codes 822, 880 through 887, and 889 are reserved for toll-free use in the future. 811 is excluded because it is a special dialing code in the group NXX for various other purposes.
[1] [2] This change in numbering format was implemented first for central office codes by 1973, which eliminated the restriction in the middle digit (2 to 9) to also permit 0 and 1. The middle position of the area code could only be 0 and 1. In 1995, this restriction for area codes was lifted as well, creating interchangeable NPA codes.
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]
This is a list of future area codes in the North American Numbering Plan (NANP) that are in the planning stages for relief of central office code exhaustion in the given numbering plan areas (NPAs). The dates are subject to change during implementation as published in the official NANP Administrator Planning Letters.
NANP members are assigned three-digit numbering plan area (NPA) codes under the common country code 1, shown in the format 1 (NPA). 1 North American Numbering Plan; 1 – United States, including United States territories: 1 (340) – United States Virgin Islands; 1 (670) – Northern Mariana Islands; 1 (671) – Guam; 1 (684) – American Samoa
Most of the Washington region's inner ring is a single local calling area, even though it is split between three area codes–the District's area code 202, Maryland's area code 301 and Northern Virginia's area code 703. For most of the second half of the 20th century, it was possible to dial any number in the metro area with just seven digits.
There are also 72 area codes available for non-geographic assignment, and 20 codes in two contiguous areas, 370–379 and 960–969, that are being held back for, I don't know, maybe in case a continious series of 10 area codes will someday be needed for something.