All the “letters to numbers” conversions refer to the time pre “phone day” (1995?) when the extra “1” was added after the initial zero.
16th April 1995. This was the second stage of three in reforming the system.
- The first, in 1990, changed the London 01 code to 071 and 081. This allowed:
- The Phoneday change in 1995, where all codes had a 1 inserted - thus 071 became 0171, 0234 became 01234 etc. Also some new codes were introduced such as 0118 for Reading (previously 0734). This freed up all codes not beginning 01xxx for further use in:
- "Big Number Change" in 2000 - London recombined as 020, and a number of other areas got 02x codes such as Cardiff (029, previously 01222). This change also renumbered some mobile phone codes so they all started 07xxx, and all premium and freephone codes in the 08xxx range (eg 0800 had previously been Tongue, in Scotland, and 0865 had been Oxford)
The point of all this was so that all numbers would have eleven digits - for example by giving Cardiff a 3-digit code (029) instead of four (0222) the number of available numbers was increased by a factor of ten.
Incidentally, although I knew that there was an alphanumeric significance to most diallg codes (e.g 0222 and 0223 were CAmbridge and CArdiff, I only recently spotted that the 01x1 codes also fit:
0121 B- Birmingham
0131 E- Edinburgh
0141 G- Glasgow
0151 L- Liverpool
0161 M- Manchester
071 was originally intended for Sheffield, which would have fit the pattern, but that eventually became 0114
091 was allocated to the Tyne & Wear area, which fits W but not T.
But I digress