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Telephone dialing procedure

Seven digit dialing is a telephone dialing procedure customary in some territories of the North American Numbering Plan (NANP) for dialing telephone numbers in the same numbering plan area (NPA). NANP telephone numbers consist of ten digits, of which the leading three are the area code. In seven digit dialing it is not necessary to dial the area code. The procedure is also sometimes known as local format or network format.

Originally, telephone exchanges consisted of manual switchboards operated by switchboard operators. Telephone numbers had typically two to four digits, depending on the size of the community. As the number of subscribers grew, multiple exchanges served individual neighborhoods of large cities. Multiple exchanges were identified by a central office name and typically four digits, such as "Pennsylvania 5000". A rural telephone number, often party line, had often up to four digits and a letter or letter and digits to indicate which of the multiple parties on the line was desired. Various methods were used to convert these to dialable numbers as dial systems replaced manual switchboards; many moderately large cities used a 2L 4N format where "ADelaide 1234" would be dialled as AD 1234 (23–1234, a six digit local call). The four largest cities, New York, Chicago, Philadelphia, and Boston, used seven digits. In New York, for example, "PENnsylvania 5000" became PEN 5000 and later PEnnsylvania 6 5000, dialled PE6 5000 or 736–5000). New York used the 3L 4N format from 1920, when dial telephones were first introduced there, until 1930, when it switched to 2L 5N. The original North American area codes were assigned in 1947 as routing codes for operator calls, but by 1951, the first cross country Bell System direct distance dial was placed directly from a subscriber station. The system was based on fixed length numbers; a direct dial long distance call consisted of a three digit area code and a seven digit local number. Numbers in 2L 4N cities (such as Montréal and Toronto) were systematically lengthened to seven digits in the 1950s, a few exchanges at a time, so that all local numbers were seven digits when direct distance dialling finally came to town. Exchange prefixes were added to small town numbers to extend four or five digit local numbers to the standardised seven digit length, matching in length the then longest local numbers in the largest major US markets.

Within the multinational calling area administered by the North American Numbering Plan, telephone numbers are composed of three fixed length fields: a three digit numbering plan area (NPA) code (area code), a three digit (NXX) central office code, and a four digit (XXXX) station number. In seven digit dialing, only the central office code and the station number is dialed, indicating that the call destination is within the local area code. This was the standard in most of North America from the 1950s onward. In some small villages with only one local exchange, it may have been permissible to dial only the four digit station number. Even after exchange names were introduced, it was possible in many small cities to call local numbers by dialing only the five digits which followed. A long distance call within the same area code could often be dialed as 1+7D, without using an area code. The scheme relied on the second digit of an area code being 0–1 and the second digit of a local exchange being 2–9. This dialing plan was incompatible with the introduction of area code 334 and area code 360, and was therefore eliminated by January 1, 1995, in the United States, and by September 1994 in Canada. It was also eliminated as early as 1981 in some numbering plan areas in the United States that had introduced interchangeable central office codes. Interchangeable central office codes are central office codes (NXXs) which, with a zero or one as the middle digit, resemble and duplicate area codes in the pre 1995 format. They were introduced to postpone area code splits in major cities such as New York City and Los Angeles, but in 1988, AT&T/Bellcore made them mandatory for area codes nearing exhaust of non interchangeable codes. The Massachusetts 617/508 split was the last one before the policy changed—617 did not yet have interchangeable NXXs at the time. Area codes with interchangeable NXXs had mandatory 10 digit long distance dialing in order to allow exchanges to distinguish between intra and inter area code calls. From 1988 to 1994, few area codes splits were possible due to the dwindling supply of area codes, so conservation measures became necessary. As of 1995, with the introduction of interchangeable NPA codes, nearly all code combinations are useful as NPAs or as NXXes.

Area code overlay complexes introduced the requirement that calls must include the area code, resulting in a ten digit dialing. Seven digit dialing remains possible in some areas of North America with a single area code. Traditionally, calling from one area code to another, specifically for long distance calls, requires the caller to dial the trunk digit "1" before the code and number. More recently, with the increasing number and decreasing geographic size of area codes, it is increasingly possible to dial a number in another area code that is not long distance where such a call does require the area code, but not the trunk digit (initial "1"). Many modern cellular phones automatically prepend the telephone's own area code if the user enters only seven digits, sending a total of ten digits. This is the same case with many landline providers that allow this. And also with many voice over IP services, users can configure default handling of seven digit dialing in a dial plan.

Local telephone numbers were governed by varying local numbering plans based on historical growth of services. Places without dial service often used special party line syntax (e. g., 2 R 48). The largest cities already used seven digit telephone numbers and had dial service.

The American Telephone and Telegraph Company (AT&T) announced a nationwide numbering plan in 1947, which divided the United States and Canada into 86 numbering plan areas and assigned a unique area code to each. All area codes had a zero or one as the middle digit. A zero indicated that the state or province was served by a single area code, and a one indicated multiple divisions of a state or province. Area codes were used as a universal destination routing code to the numbering plan area, and replaced the trunk routing codes that operators previously had to look up at the toll switching centers under the General Toll Switching Plan that was in use since 1929. In Operator Toll Dialing, automated equipment translated area codes to routing information. Systematic conversion of city dial systems commenced to the seven digit (two letter five number) numbering plan. Central office codes were restricted to the digits 2 to 9 in the middle position, to facilitate machine recognition when an area code was dialed. The same rule already applied to the first digit for technical and historical reasons.

Direct Distance Dialing (DDD) trials were conducted by the end of 1951, in which subscribers in Englewood, New Jersey, could dial long distance calls to a select group of remote destinations, as far away as San Francisco, by using an area code. DDD technology expanded to other major cities, By 1960, it was available in a few places in Canada, as well as most large American cities. This decade is notable for implementation of some thirty more area codes, including in Alaska, Hawaii, and the Caribbean.

Major progress may be noted in provisioning DDD service. Long distance billing was computerized in the 1960s to the early 1980s. Few area codes were introduced during this time. Toll free 800 service was introduced. Service demands in the largest American cities of New York and greater Los Angeles area resulted in the first use of interchangeable central office codes (NXX). All number calling was implemented, replacing 2L 5N telephone numbers. A very small number of places still did not use a seven digit numbering plan by 1981.

New area code assignments were increasing in the 1980s. From 1990 to 1994, all remaining assignable codes entered service. The exhaustion of NXX codes necessitated interchangeable codes in several more area codes. Ten digit dialing, or 1+10D, was implemented in area codes with interchangeable NXX codes. Protected dialing plans as in national capital areas were discontinued to help meet demand without area code relief. As 1994 neared its end, ten digit dialing became required throughout the numbering plan in preparation for interchangeable NPA codes. All local numbers now had seven digit, as the last technological hold outs had given way to modern switching technology. The concept of a ten digit local number was conceived, as New York had an overlay code (917, implemented in 1992), but seven digit dialing was still the norm.

On July 16, 2020, the FCC adopted rules to establish 988 as the three digit phone number for the National Suicide Prevention Lifeline. This required 82 area codes to switch to mandatory ten digit dialing by October 24, 2021, because they had already used 988 as a central office code. Similar transitions are scheduled in most parts of the remaining single code areas in Canada by May 31, 2023, due in part to the rollout of 9 8 8 service in that country.