Ten-digit dialing

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Ten-digit dialing is a telephone dialing procedure in the countries and territories that are members of the North American Numbering Plan (NANP). It is the practice of including the area code of a telephone number when dialing to initiate a telephone call. When necessary, the ten-digit number may be prefixed with the trunk code 1, which is referred to as 1+10-digit dialing or national format.

History[]

The implementation and expansion of the North American Numbering Plan between 1947 and 1992 preserved a long-standing practice in the United States and Canada that callers should only need to dial the local seven-digit telephone number when placing a call within the caller's exchange area or within the home numbering plan area (NPA). In seven-digit dialing. callers dialed the central office code (three digits) and the four-digit station number of the telephone subscriber to reach in the same numbering plan area. Dialing of an area code before the telephone number, referred to as ten-digit dialing, was only necessary for foreign NPA (FNPA) calls.

Some communities with significant parts on both sides of an area code boundary implemented central office code protection, where possible, to ensure the same seven-digit local number was not assigned in two different area codes in the same city to assure seven-digit dialing in the entire community.

Code exhaustion relief[]

When the demand for telecommunication services in a numbering plan area threatened to exceed the capacity of telephone numbers of a single area code, the local public service agency and the Federal Communications Commission initiated relief measures to mitigate the exhaustion, and assure continued service with minimal customer impact in the affected areas.

Area code split[]

The established practice of relief from 1947 to 1992 was to divide the affected numbering plan area into suitable parts, most often two, and assigned a new area code to one of the areas, while maintaining the existing area code in the other, most commonly the denser-populated or geographically or historically more significant part. This freed the telephone numbers already issued in the new area for use in the existing, now smaller, numbering plan area. This practice preserved seven-digit dialing in all affected areas.

Area code overlay[]

By 1992, a new method of relief became available from the proliferation of electronic switching systems that were based on stored program control (SPC), which reduced the need for electromechanical extensions of the switching fabric. This made it possible to assign multiple area codes to a single numbering plan area. An area code overlay required additional technology in all affected switching systems to recognize the local area codes and select the appropriate routing for local calls, even within the wire center. This eliminated the possibility of maintaining seven-digit dialing, but required the dialing of ten digits for all calls.

Consumer groups and some state regulators, such as the Illinois Commerce Commission and Citizens Utility Board for northwest Chicago, and the NYS Public Service Commission in NYC, opposed the requirement with attempts at litigation.^[1][2][3]

The added effort in remembering and dialing longer numbers damaged the popularity of overlay plans,^{[citation needed]} which were introduced as a means to reduce the inconveniences associated with the traditional method of splitting an area. However, overlay plans have become the primary relief method and ten-digit dialing has become increasingly common in the U.S. and Canada.

1+10 dialing[]

Long-distance calls within a numbering plan area may also require dialing of an extra digit. 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.

Dialing 1 before the area code is most often required only for long-distance calls. Some telephone systems in early overlay areas do not accept a "1" before the area code for local 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 for landline phones. All cellphones in North America ignore this and only require the user to dial ten digits.^[4]

Ten-digit dialing in non-overlay areas[]

A few areas that are not within an overlay plan nonetheless require ten-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 ten-digit dialing for all off-base numbers when the 270/364 overlay was established in 2014.^[5]

Area codes (in blue) affected in the lower 48 states, not including Alaska (907), Guam (671), and Hawaii (808)

A July 2020 order by the Federal Communications Commission required ten-digit dialing of telephone numbers with area codes in the United States that were not overlaid, but had an assignment of the prefix 988 as a central office code, conflicting with its designation for the National Suicide Prevention Lifeline by federal law.^[6]

NANPA Planning Letter 544 cites 83 area codes affected in 37 states. Three additional area codes were already transitioning to ten-digit dialing as the result of overlay implementations. Central office code 988 was in the process of discontinuation for area code 904.^[7] While included in the list of 83 area codes, area code 701 in North Dakota will not be converted to ten-digit dialing because the 32 telephone numbers in the 988 central office were reassigned to another prefix.^[8] The implementation schedule determined a permissive dialing period in the affected area codes from April 24, 2021 to October 24, 2021, ahead of the deadline of July 16, 2022, for all calls to 988 to be routed to the lifeline.^[7]

See also[]

• List of country calling codes
• Telephone numbering plan

References[]

External links[]

• NANPA report of NPAs requiring ten-digit dialing