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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). NPAs with 9 in the second position are reserved for future format expansion.
The largest telephone numbering plan in North American is the North American Numbering Plan (NANP), serving 25 regions or countries. Other countries maintain an autonomous numbering plan with distinct country codes within the international E.164 specifications by the International Telecommunication Union.
This makes the fastest-dialing area code 212 (5 total clicks), followed by 312 and 213 (6 clicks). The area code 605 is much slower than these, at 21 clicks. Many large US cities generally had fast-dialing area codes with the middle digit 1. Touch-tone dialing, introduced later, eliminated the wait.
Using 0 or 1 as the first digit of an area code or central office code is invalid, as is a 9 as the middle digit of an area code; these are trunk prefixes or reserved for North American Numbering Plan expansion. For example, 234 235-5678 is a valid telephone number; with area code 234, central office prefix (exchange) 235, and line number 5678.
Numbering plan areas in California (blue) and border states. This map is clickable; click on any region shown to visit the page for those area codes. In 1947, the American Telephone and Telegraph Company (AT&T) devised the first nationwide telephone numbering plan and assigned the original North American area codes.
Area code 303 is the original area code, while area codes 720 and 983 were added (in 1998 and 2022 respectively) to form an overlay complex for the area, meaning that the same geographic service area is served by all area codes and that ten-digit dialing is required for all calls made within the service area.
Area code 708 was created on November 11, 1989, in a split of area code 312. It once served almost all of Chicago's suburbs in Illinois. In 1996, its numbering plan area was reduced in a three-way area code split. The northern suburbs received area code 847, while the western suburbs were assigned area code 630.
Area code 209 was created in an area code split of area code 415 on October 26, 1957. On November 14, 1998, the southern half of this numbering plan area received the assignment of area code 559. On October 24, 2021, 209 was transitioned to 1+10-digit dialing despite not being part of an overlay complex at that time.