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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.
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 telephone number when placing a call within the caller's numbering plan area (NPA). In seven-digit dialing. callers dialed only the central office code (three digits) and the four-digit station number of the telephone subscriber to reach in the same numbering plan area. For example, a person at a station with the telephone number 212-555-7890 called the number 212-555-3456 by dialing 555-3456.
Dialing of an area code before the telephone number was only necessary for foreign NPA (FNPA) calls. This is referred to as ten-digit dialing.
Some communities with significant parts on both sides of 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, where possible, 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.
Some switching systems require the caller to dial 1 first as a trunk prefix before the area code and number, to indicate that the call requires a connection to another area, or that the call is a toll call.
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 with overlays.
Area code splitEdit
When the demand for telecommunication services in a numbering plan area threatened to exceed the capacity of telephone numbers of a single area code, it was the established practice from 1947 to 1992 to divide the effected 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. This practice preserved seven-digit dialing in all effected areas.
The introduction of overlay plans as a means of mitigation of exhaustion by 1992, broke the possibility maintain seven-digit dialing. In an area code overlay, multiple area codes are assigned to the same geographic area.
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 dialing for all local calls 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 for landline phones. 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.
Introduction of ten-digit dialing in non-overlay areasEdit
While most ten-digit dialing areas are also overlay areas, a July 2020 order by the Federal Communications Commission will result in the expansion of 10-digit dialing (or 11-digit dialing for California landlines) to dozens of area codes that are not overlaid, have one area code, and had previously accepted seven-digit dialing. This is because these area codes have the number 988 as a central office code, potentially conflicting with the designation of 988 as a three-digit code for the National Suicide Prevention Lifeline.
NANPA Planning Letter 544 cites 83 area codes that are affected in 37 states; three additional area codes were already transitioning to ten-digit dialing as the result of overlay implementations, and the 988 central office code was discontinued for area code 904. While included in the list of 83 area codes, area code 701 for North Dakota will not implement ten-digit dialing but reassign the 32 telephone numbers in the 988 central office to another. Per the schedule adopted by NANPA, a permissive dialing period in the effected area codes has been scheduled 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.
- "The 10- or 11-Digit Local Call Fosters Anxiety and Shrugs". NY Times. 23 January 2003.
- "Days are numbered for 7-digit dialing". Chicago Tribune.
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- Schock, Victor (2020-12-18). "Re: Implementation of the National Suicide Hotline Improvement Act of 2018" (PDF). North Dakota Public Services Commission. Retrieved 2020-12-22.