Seven-digit dialing


Seven-digit dialing is a telephone dialing procedure customary in the territories of the North American Numbering Plan, for dialing telephone numbers in the local calling area. The procedure is also sometimes known as local format or network format.

History

Originally, telephone exchanges consisted of manual boards operated by switchboard operators. Numbers were typically four digits or fewer for local calls within an exchange due to practical limitations. As the number of subscribers grew, multiple exchanges served individual neighborhoods of large cities. A city telephone number consisted of an exchange name and four digits, such as "Pennsylvania 5000". A rural telephone number, often party line, was often up to four digits plus a letter or letter and digits to indicate which of the multiple parties on the line was desired.
Various schemes were used to convert these to dialable numbers as dial replaced manual switchboards; many moderately-large cities used a 2L+4N format where "ADelaide 1234" would be dialled as AD-1234. A few of the largest cities, such as New York, used seven dial pulls.
The initial 86 area codes were assigned in 1947 as routing codes for operator calls; the first cross-country Bell System direct distance dial call was made in 1951. 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 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.

Structure

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 code, a three-digit central office code, and a four-digit 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.
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 also was 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 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.

Office code protection

Traditionally, identical central office codes in adjacent Numbering Plan Areas 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:
Area code overlay plans introduced a requirement in most areas that calls must include the area code, resulting in a 10-digit dialing requirement. Seven-digit dialing remains possible in some areas of North America.
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.
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.

Timeline

Before 1947

Local numbers were of varying length depending on local provisioning of service and the technology in place. Places without dial service varied between those with "modern-looking" numbers such as an exchange name and digits and those with numbers that referenced a multi-party line identification. New York City had the longest numbers, as two-letter-five-number.
The North American Numbering Plan was implemented in 1947, on paper, assigning 86 area codes to approximate areas of 48 states, D.C. and nine provinces. All area codes had a zero or one as the middle digit.

1947 to 1951

The NANP area codes were implemented in use to allow operators to dial other operators for call completion assistance. Several cities were upgraded in this period to seven-digit phone numbers. As there are no letters on the 0 or 1 position, it fit with the assumption that no central office code would have a zero or one as the middle digit.
Direct Distance Dialing was implemented in late 1951 in Englewood, New Jersey, giving customers in that exchange the ability to directly dial phones in a select number of cities as far away as San Francisco.

1951 to 1960

Direct distance dialing slowly was expanded to other major cities, restrained by the complexity of the technology, the time needed to expand local exchanges to seven-digit numbers, and the limited technology to compile and process billing of long-distance calls without modern computers being affordable. By 1960, a few places in Canada had DDD as well as most large American cities. This decade is notable for some thirty more area codes being implemented, including codes for Alaska, Hawaii and the Caribbean.

1960 to 1981

Major progress in provisioning DDD service, computerizing long-distance billing. 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 resulted in the first use of interchangeable NXXes. All-number calling is implemented, replacing 2L-5N numbers. Although a very small number of places still have non-seven-digit dialing by 1981, they are extremely rare.

1981 to 1994

New area codes are introduced, a handful in 1981–83, then status-quo for most of the time until 1990. From 1990 to 1994, all remaining assignable codes are put into service. Exhaust of NXX codes results in interchangeable NXXes in several more area codes. 1+10 digit or 10-digit dialing is implemented in area codes that have interchangeable NXX codes. Protected dialing plans as in national capital areas are discontinued to help meet demand without an area code relief. As 1994 nears an end, 1+10 or 10 becomes required throughout the numbering plan in preparation for interchangeable NPA codes.
All local numbers are now seven-digit, as the last technological hold-outs have given way to modern switching technology. The concept of a ten-digit local number is now conceived, as New York has an overlay code, but seven-digit dialing is still the norm.

1995 to present

Interchangeable NPA codes are introduced in Washington and Alabama, and some 40 new area codes are introduced through the year 1999 as pent-up demand for code relief is implemented. This includes two additional toll-free prefixes as the 888 code was quickly exhausted by assorted wasteful practices. Since 1999, a more steady rate of area code introductions has taken place, the rate being slower due to one or more factors:
With overlays in several areas, ten-digit local numbers are now supplanting seven-digit dialing; as of 2019, only four Canadian area codes are still single-code areas and allow seven-digit local dialing. Although fewer American area codes are overlaid, seven-digit dialing is also disappearing in the United States.