Seven-digit dialing

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Seven-digit dialing is a popular term referring to the traditional convention in the United States and Canada for dialling local phone calls. It is also sometimes known as local format or network format.

History[edit]

The original telephone exchanges were 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 neighbourhoods 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[edit]

Within the multinational calling area administered by North American Numbering Plan, telephone numbers are segmented into fixed-length fields:

  • 1 plus:
  • a three-digit (NXX) exchange, indicating (amongst others) a city or other municipal area
  • a four-digit (XXXX) station number

Seven digit dialling is when only the subscriber service number is dialled, without the area code. This was the standard in most of North America from the 1950s onward; in a few small villages where no other exchange was local, a four-digit call remained possible within the village if the last four digits of a local subscriber line were numbered 2000-9999 and any call outside the individual exchange required a leading 1- or 0 as a toll call. From outside the exchange, these were still 1+7D as the exchange code and four-digit number still composed the standard seven digits.

The multiple individual subscribers on party lines were assigned separate individual, standard length numbers; suffixes like "line 123, ring two" were dropped.

In most areas, a long distance call within the same area code could be dialled as 1+7D, with no need to dial the area code, until 1994. 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 was broken by the introduction of area code 334 and area code 360 in 1995.

Before the advent of overlay plans, it was universally accepted (and in some cases, required) that a call to a local number in the same area code as the calling station be placed without including the area code. As a result, the caller has to dial only the 7 digits of the exchange plus the station number.

Code protection[edit]

Traditionally to avoid number confusion, identical exchange numbers in different area codes would be assigned as far apart from each other as possible, so that callers living near a state or NPA boundary would not get two areas in different NPAs confused. This "exchange code protection" made it possible in some low or moderate-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 613 and 819).[1]

Limitations[edit]

This convention did not have a name until overlay plans introduced a requirement in some areas that all calls, even local, must include the area code, i.e. 10-digit dialing. Traditional 7-digit calling is still valid in those portions 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 which 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 will automatically prepend the phone's own area code if the user enters only seven digits. When the caller dials only seven digits, the number sent to the network is actually ten digits. Voice-over-IP users can configure default handling of seven-digit calls at the analog telephone adapter if the device settings have not been locked by a provider; a default area code for seven-digit local calls can also be configured in software such as Asterisk PBX locally or at an upstream provider.

See also[edit]

References[edit]

  1. ^ In-state calling (archived) at fairpoint.com