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.