Linked numbering scheme

A Linked Numbering Scheme (LNS) is a telephone numbering plan applied to an area of the country where calling between lines on a number of adjacent exchanges is done without using a dialing code.

United Kingdom

The largest LNS in the UK is that for the London telephone area - formerly known as the London Director area. Here, several million subscribers can call each other by dialing a uniform code. Thus, for example, anyone calling from an (020) number can reach Transport for London travel enquiries by dialing 7222 1234.

Smaller schemes apply outside London. Uxbridge, for example, has the subscriber trunk dialing (STD) code 01895. Uxbridge exchange is the parent for Denham, Harefield, Ruislip and West Drayton; anyone connected to any of those exchanges can call any of the others without having to prefix the number with 01895. This is achieved by giving subscriber lines on each exchange different prefix numbers, thus: all numbers are six-figure; Denham numbers start with 83, Harefield with 82, Ruislip with 6 and West Drayton with 4. Uxbridge numbers start with 2 or 81. All calls must have all six digits dialled - even if a subscriber is on Denham exchange and is calling another subscriber on Denham exchange, they must still dial 83xxxx.

Incoming calls from any other exchange for a subscriber on any of the five exchanges must all be prefixed with the same 01895 code.

North America

The terminology "linked numbering scheme" is not used in the North American Numbering Plan as all numbers are constructed with +1, then a fixed-length area code (three digits) which covers not one exchange but all or a large part of a state or province. This is followed by seven more digits, of which the first three identify an individual local exchange.

It is presumed that each area will, by default, contain many different exchanges in different towns (or different parts of a large city). A local call in this scheme, standardised for the 1951 introduction of direct-dial long distance calling, was originally dialled as a fixed-length seven-digit number. A "local call" does not infer "same area code" (or vice-versa) and the local (usually flat-rate) calling area of all but the most isolated villages would contain multiple exchanges and one or more adjacent towns.

Canada

Exchanges on an area code boundary (the largest being Ottawa-Hull due to its location on a provincial border) originally used a code protection scheme where an exchange prefix in use on an area code boundary would, upon assignment in one area code, be reserved as unavailable to anyone in the other. Code protection ensured that no two local numbers would have the same exchange, even in different area codes, allowing any local call across the boundary to be uniquely completed with seven digits.

As the reservation was at the area code level, any prefixes in use in Ottawa (area code 613) would be unassignable not only in nearby Hull (area code 819) but in all other +1-819 cities, including distant Sherbrooke, Quebec and even James Bay - both in the 1-819 area code but not local to anything near Ottawa-Hull.

In cases where a village on an area code boundary had only one exchange of 10000 or fewer numbers, the inherent losses of available numbers under code protection were negligible enough to be ignored. Ottawa-Hull, however, has over a million people and shares area codes with many other cities hundreds of kilometres distant.

This unnecessarily-wide reservation (along with other numbering inefficiencies, such as allocation of numbers in blocks of 10000 to each competing local carrier in even the smallest towns) led to premature depletion of available numbers in both area codes, causing the established seven-digit dial plan to break down and code protection to be scrapped entirely not only in Ottawa-Hull but in every town which shared any area code with that city. The end result was to force tiny towns like Beebe Plain (which is divided by the Quebec-Vermont border and formerly could make a seven-digit local call internationally within the village) to dial ten digits even if the called subscriber is elsewhere in the same building.

Dual dialability (where a subscriber is assigned the same seven-digit number in both area codes, duplicating entire exchanges) is rare; the one ongoing instance of its use is for Canadian federal government offices in +1-819 Hull where the corresponding numbering resources in Ottawa's +1-613 are wasted by listing the entire Hull federal civil service duplicatively in both area codes.

United States

In the United States, from the creation of the Washington, D.C. area code 202 in 1947 until 1991 when the number of telephone lines exceeded the capacity of one area code, all persons in the City of Washington as well as the adjoining communities in the U.S. States of Maryland and Virginia, could call any number in the region by simply dialing the 7 digit local number, even if the number was in a different state. The way this was done was, in the adjoining counties in Maryland and Virginia, in addition to the area code 703 or area code 301 numbers, they would also be assigned the same local number in the 202 area code. This meant a 585-xxxx number in Silver Spring, Maryland would be reachable from outside the area either as a 202-585-xxxx or 301-585-xxxx, but would be called from DC or Virginia as 585-xxxx. It also meant that any prefix (such as 585 in the above example) in the entire region could not be duplicated.

In some other rural areas in the U.S. where local calling areas cross state lines, numbers in adjoining areas could be called by dialing the 7-digit number without the area code. If the same prefix existed in the caller's area code, they would have to dial it as a long-distance number.