Linked numbering scheme

A linked numbering scheme (LNS) is a telephone numbering plan in effect in a service area within which call routing between subscriber lines on adjacent exchanges does not require a dialing code.

United Kingdom

The largest linked numbering scheme in the UK is that for the London telephone area, formerly known as the London Director area. Within the area, several million subscribers can call each other by dialing a uniform code. 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 (NANP) as all numbers are constructed from a 3-digit area code which covers not one exchange but all or a large part of a state or province, followed by the 7-digit subscriber number, of which the first three digits identify the telephone exchange to which the subscriber loop is connected.

Each area may contain many exchanges in different towns or districts. 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.

The seven-digit dialling scheme for local calls in many areas bears partial similarity to the linked numbering scheme, but (because a North American area code is a wide region, most often not a single city) not all calls within an area code are necessarily local or dialable as seven digits. In some cases, a number just across an area code boundary may be seven digits as part of an exchange code protection scheme if it's a local call and if available unique local exchange prefixes have not been exhausted. Any overlay plan will break this for some or all calls, forcing ten-digit dialling.