This article needs additional citations for verification. (December 2009) (Learn how and when to remove this template message)
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 referred to as 1+10-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 dialed only when the area code of the called number is different from 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.
The phone system often requires the caller to dial 1 first as a trunk prefix before the area code and number, to indicate to the phone system that the call requires a connection to another area. "1" is also the country code for the North American Numbering Plan, and therefore must likewise be dialed before the area code for international calls made to NANP countries.
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 numbering plan areas since that time have 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 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 ten-digit dialing, even where a call is seven-digit local.
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 response to pressure from carriers, the U.S. Federal Communications Commission and the Canadian Radio-television and Telecommunications Commission (CRTC) have imposed 10-digit dialling for all local calls (even within the same area code) in the overlaid areas. The requirement has no technical basis, but carriers expected it would reduce objections from new subscribers assigned the less-desirable overlay code by inconveniencing everyone equally. Consumer groups and state regulators (the Illinois Commerce Commission and Citizens Utility Board for northwest Chicago, the NYS Public Service Commission in NYC) pushed back against the requirement with attempts at litigation, to no avail. The requirement is unenforceable against PBX vendors and voice over IP operators as the dial plan is controlled by subscriber-owned equipment, which can be configured to send seven-digit calls to the original area code. It has also failed to stop a pattern of some subscribers paying third-party resellers an artificially-high price for a number in a desirable original area code like Manhattan's 212 or Toronto's 416. A business which advertises a main number in a random overlay which did not exist at the turn of the millennium marks itself as a newcomer, or even as someone doing business from a mobile telephone, placing it at a disadvantage against long-established local competitors who first opened their doors in an era when there was just one telephone company and one area code.
The "1" before the area code is most 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; all Canadian landlines follow this pattern. However, in the three largest US markets (New York City, Los Angeles and Chicago), the initial "1" is required even for local calls. All cellphones in North America ignore this and only require the user to dial 10 digits.
The added dialing requirement, coupled with the need to remember which of the area's coincident area codes applied to a seven-digit local 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, most 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 ten or eleven digits. Eleven digits for toll calls 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.
A few areas that are not within an overlay plan nonetheless require 10-digit dialing if part of the local calling area is served by an overlay plan. One example of such an area is the Fort Knox Army base in Kentucky. The base itself is served by area code 502, which is not yet subject to an overlay plan, but its local calling area includes cities that are now served by the 270/364 overlay complex. As a result, Fort Knox imposed 10-digit dialing for all off-base numbers when the 270/364 overlay was established in 2014.