Seven-digit dialing

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.

Traditionally, identical central office codes in adjacent Numbering Plan Areas (NPAs) would be assigned as far apart from each other as possible, so that callers living near an NPA boundary would not confuse numbers in the adjacent NPAs. Central office code protection made it possible in some low density areas to use seven digits to reach areas in another area code.

Examples:

• Washington, D.C. (area code 202) to adjacent areas of Maryland (area code 301) and Virginia (area code 703), discontinued on October 1, 1990 to allow assignment of a code in all three area codes.
• Hull from Ottawa (before 2006, as every Ottawa-Hull local number originally was reserved in both area code 613 and 819).[1] This was terminated on 21 October 2006 to allow assignment of a code in both 613 and 819, although five central office codes associated with the federal government were protected in 819 to allow dialing without potentially reaching non-government lines.

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.

• Linked numbering scheme
• E.164