[1] The change affected subscriber numbers in the cities of Birmingham, Edinburgh, Glasgow, Liverpool, London and Manchester which used the Director telephone system.
In six metropolitan areas around major cities, groups of multiple exchanges operated in a director telephone system.
The General Post Office issued subscriber trunk dialling (STD) codes for most areas during the late 1950s and early 1960s.
As STD was expanded to non-director areas, every telephone which had been issued with no letters on the dials needed to be altered for alphanumeric dialling.
[2] The General Post Office produced a pamphlet All-Figure Telephone Numbers in 1965 which summarised the problems.
At the same time, many local exchange codes were altered in order to group them into geographic sectors (e.g. GROsvenor became 499 rather than 470) to eliminate the need for all incoming calls to be routed via central London.
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was the first to withdraw the ability to dial the old codes in January 1970 and from April the parallel operation was withdrawn in the other five cities.