Changes
From 1966 telephone area codes ceased to be represented with letters. In most areas this did not require any changes to the telephone codes or numbers. However, in director telephone areas such as the London telephone area, most local exchange codes were altered in order to group them in to sectors. The change permitted new area codes to be issued that did not correspond to letters representing the location.
In the director areas the old codes continued to work in parallel with new codes until 1970 when the "ANN: All-figure Numbers Now" advertising campaign prompted callers to dial only the new codes. By October 1969 in Edinburgh 79% of calls were being made using the new local exchange codes, in London 72% and in Glasgow 43%. London was the first to withdraw the ability to dial the old codes in January 1970 and from April the parallel operation was withdrawn in the other five cities.
The change to all-figure dialling was controversial and was debated in both Houses of Parliament. Among the criticisms of the new system was the perceived difficulty in remembering telephone numbers, the inability to easily recognise where a telephone number was located and the need for a change so soon after the implementation of subscriber trunk dialling.
Read more about this topic: All-figure Dialling