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. It is also sometimes known as local format or network format.
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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 (if each line had a jack on the switchboard, four digits or 10000 possible numbers filled a 100 x 100 board). 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 (23-1234, a six-digit local call). A few of the largest cities, such as New York, used seven dial pulls ("PENnsylvania 5000" became PEN-5000 and later PEnnsylvania 6-5000, dialled PE6-5000 or 736-5000).
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 (such as Montréal and Toronto) were systematically lengthened to seven digits in the 1950's, 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 (NPA) code (area code), indicating a geographical area, 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.
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 broken by the introduction of area code 334 and area code 360 in 1995.
Office code protection
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, such as Hull from Ottawa before 2006, as every Ottawa-Hull local number originally was reserved in both area code 613 and 819.
Limitations
Area code overlay plans introduced a requirement in most areas that all calls, even local, must include the area code, resulting in the term 10-digit dialing. Seven-digit calling is still possible in those numbering plan areas of North America not subject to an overlay plan.
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 call 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").
An individual area code can accommodate about 7.8 million numbers if used efficiently (based on seven digits, restricted so that a local number cannot begin with 0 or 1, N-1-1, or a reserved code like 950, 958, 959 or an exchange prefix duplicating an in-region area code). This may not be enough for New York City (pop 8,405,837 in 2013) if every resident wants their own mobile telephone; multiple area codes will be needed and seven-digit calls between area codes will inevitably break.
There is no legitimate technical need to require that a call within the same area code use more than seven digits, although telephone companies in both the US and Canada have successfully lobbied federal regulators to impose this arbitrary requirement on the assumption that no new subscriber would want a ten-digit local number while existing subscribers in the original area code could still reach other with seven-digit local calls.
In theory, an area code that covers two large but distant cities, such as Windsor, Ontario and London, Ontario in area code 519 (175 km apart), could be overlaid without breaking any seven-digit local calls if exchange prefixes are carefully assigned so that no two numbers with the last seven digits are placed in the same town's local calling area and if long distance calls require 1+area code before the number. In practice, this is never done; in one-town area codes, such as 212 (Manhattan), it is not possible.
If the subscriber controls the dial plan, there is no way to force the use of ten digits within the home area code. 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. In many voice-over-IP services, users can configure default handling of seven-digit dialing.