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 dialing local phone calls. It is also sometimes known as local format or network format. Every area in the United States and Canada has switched to 10-digit dialing.

Seven digit dialing is when only the subscriber service number is dialed.

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

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

As recently as 1970, and possibly later, calls within small towns in Canada required only the last 4 digits. The local network was like a PBX, with each number effectively an extension.

Earlier, in the 1940's and 50's, many American homes were on "party lines". If one picked up the phone and someone was talking, one had to try later, until the line was free. The musical play, "Bye Bye Birdie," used this device to portray teenagers in a small town gossiping together.

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").

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]

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.

It is possible to make a long distance call within the same area code; in this case the caller had to dial a "1" before the local 7-digit number if the leading "1" was used for toll alerting (8-digit dialing). In many areas, before 1995, including the area code on a toll call within the same area code would confuse the telephone system and prevent the call from being connected. Today, all long distance calls require 11-digit dialing, even within the same area code, if toll alerting is in place.

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.

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 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.

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