When Subscriber Trunk Dialling (STD) was introduced in the early 60s dialling codes for the non-Director
* exchanges were written as:
zero, two letters with mnemonic significance followed by one, two or three digits, e.g.:
BRistol 0
BR2 (
0272)
CArdiff 0
CA2 (
0222)
IPswich 0
IP3 (
0473)
Keynsham (
BRistol) 0
BR5 6 (
0275 6)
etc.
The use of letters was dropped within a few years but in this case there were no actual code changes except that a few codes which had been put on 0
Oxx were changed to leave 00 free for other things (eventually International dialling). Eg. 0
OX2 Oxford became 0865.
*Six Director Cities were given seven digit telephone numbers. These were written as three letters and four digits. One of the most famous was the number for the Metropolitan Police:
WHItehall 1212 (944 1212).
About 1966 the letters were abandoned in favour of all digit numbers.
http://www.rhaworth.myby.co.uk/phreak/tenp_uk.htm