In the United States and Canada, 10-digit dialing is a popular term used to refer to the practice of including the area code of a phone number when dialing. It is also sometimes known as national format.

Traditionally, after the advent of NPAs (area codes), the phone system would allow callers to only dial the local portion of the phone number they wanted to reach, specifically as long as the called number was in the same area code as that of the caller. For example, a person whose full national phone number was (123) 456-7890 could call a number located at (123) 222-3456 by simply dialing the last seven digits. The phone system would infer that the desired number was in the same area code, and connect the call accordingly.

In this period, it was only necessary to dial the area code for a domestic call when the area code of the called number was different than that of the calling number. In this case, the phone system required the caller to dial a "1" before the area code and number, to indicate to the phone system that the call will require a connection to another area. ("1" is also the country code for the United States and Canada, and therefore must likewise be dialed before the area code for international calls made to these countries.) Typically such calls were long distance calls. For many years, a popular misconception was that a call to a different area code was always a long distance call, but the significant growth in the number of area codes (and the shrinking of the areas they occupy) since that time has invalidated this assumption.

The introduction of overlay plans as a means to reduce the need for phone numbers to change as a result of adding new NPAs meant that one geographic area could be associated with more than one area code. As a result, the phone system could not assume the area code of a 7-digit called number based on the area code of the calling station. Therefore, the introduction of an overlay NPA to an area makes 7-digit dialing impossible, and 10-digit dialing becomes a requirement (though not always with a preceding "1") for all calls.

The added dialing requirement, coupled with the need to remember which of the area's concurrent NPAs applied to a given number, damaged the popularity of overlay plans, which themselves were introduced as a means to reduce the inconveniences associated with the traditional "split" plans.

As overlay plans have spread to more areas, 10-digit dialing in the U.S. is increasingly becoming the norm, but is not yet universally required.

See also

• numbering plan area
• NANPA
• List of country calling codes