Ten-digit dialling

In the United States and Canada, ten-digit dialing is the practice of including the area code of a telephone number when dialing to initiate a telephone call. When necessary, a ten-digit number may be prefixed with the trunk code 1, which is often referred to as 11-digit dialing or national format.

After the implementation of the North American Numbering Plan, placing a local call within the caller's area code involved only seven-digit dialing. Callers dialed only the central office code (three digits) and the four-digit station number of the telephone subscriber to reach. For example, a person at a station with the telephone number 212-555-7890 called the number 212-555-3456 by dialing 555-3456.

In seven-digit dialing the area code is only dialed when the area code of the called number is different than that of the calling number. Some communities on an area code boundary, such as Ottawa-Hull (613/819), Kansas City MO/KS (816/913) or Washington, D.C. (202) implemented central office code protection to ensure the same seven-digit local number was not assigned in two different area codes in the same city to retain seven-digit dialing in the entire community. Code protection is not possible for calls across area code boundaries within split plan cities where area codes have been added due to a shortage of available local numbers; these local calls became ten digits when the code was split.

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