You must be a youngster. Before that, in the London area at least, it was three letters followed by four figures, and the old dials had both letters and numbers on them. The letters may or may not have corresponded to the area but they were always part of a memorable word, so Mitcham had MIT, but Sutton had VIG which was referred to as "vigilant" when you gave someone a phone number. I guess SUT was used for a different Sutton, Sutton Coldfield perhaps.
Not really, my mother had worked as a GPO telephonist and ever since then I'be always known several local exchanges' codes by:
a) their names in the case of directors areas and
b) their locality by their STD code as derived front the original letters when trunk dialling was first introduced.
Some samples from memory - especially when we went auto and had to dial the ourselves:
In the Ilford area there were CREscent (550), VALentine (554), CLOcktower (552), SILverthorn (529) and of course ILFord (478). Some of the central London alpha names even had an international fame, probably the most famous being WHItehall, PADdington, WATerloo and TERminus (Kings Cross). When Hainault went auto, it was given 500, (as my mother would curse, callers elsewhere in London would dial 424, which of course was the code for CHIswick.
When I moved out of the area, the national STD letter codes were quite useful: 0245 Chelmsford was 0CH5, 0206 was Colchester 0CO6, Portsmouth was 0PO5, Southampton was 0SO3 and St Albans was 0SA7. Interestingly Hatfield and Welwyn were 0PO7 despite them being some way from Potters Bar.
The Director areas were as above but with Manchester, not Newcastle. ...
Your are correct re Manchester which was given the code 061 in the '60s. Code 091 was earmarked for the north-east metropolitan area in Sunderland and Newcastle but not used until the late '80s, eventually becoming 0191 on 'phone day'.