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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. Sometimes (see below), an initial "1" is used; such dialing is known as 11-digit dialing or national format.
Traditionally, after the advent of area codes, the phone system allows callers to dial only the local portion of the phone number they wanted to reach, 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 212-555-7890 was able to call a number located at 212-555-3456 by simply dialing 555-3456. The phone system infers that the desired number was in the same area code, and connects the call accordingly. This is now known retrospectively as 7-digit dialing.
In this case, it is only necessary to dial the area code for a domestic call when the area code of the called number was different from that of the calling number. The phone system requires 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, as trunk prefix.
"1" is also the country code for the North American Numbering Plan including 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. It used to be that a call to a different area code was a long-distance call, with rare exceptions where a city falls on an area code boundary, 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.
In Canada and some regions of the United States, placing a landline call with "1" before an area code where the outgoing call is in the same service area results in an automated recording indicating that the call being made is local. The "1" toll prefix is not necessary, even if the area codes are different. This is common in areas where overlays are being used. Landline providers have warned that dialing "1" when it is unnecessary could result in long distance charges being made even when they otherwise would not have been charged.
Cellular telephones have always accepted 1-NPA-NXX-XXXX as an eleven-digit call, even where a call is seven digit local, as dialling while at the edge of the local coverage area would otherwise be unpredictable as the handset is handed off between local and long-distance towers.
The introduction of overlay plans as a means to reduce the need for phone numbers to change as a result of adding new area codes meant that one geographic area could be associated with more than one area code. This is disadvantageous to new service providers as existing providers can issue numbers in the familiar area code. In order to promote fairness among carriers, 10-digit dialing became an FCC requirement; though the "1" before the area code is often required only for actual long distance calls. Some phone systems in early overlay plan areas still do not accept a "1" before the area code for non-long-distance calls.
The added dialing requirement, coupled with the need to remember which of the area's coincident area codes applied to a 7-digit 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. and Canada is becoming increasingly common. However, areas not within an overlay plan can still use 7-digit dialing for local calls, although long distance calls within the area code may have required 10-digit dialing. This requirement became standard in all of North America by the end of 1994 to allow introduction of "interchangeable NPA codes" – area codes that did not have a 0 or 1 as the middle digit and could therefore be confused with the central office code – after January 1, 1995.
As a number format
As the familiarity of overlay plans and 10-digit-dialing required areas has increased, the phrase "10-digit dialing" is now sometimes used to describe the normal inclusion of area code in long-distance dialing.