The letter codes on old UK phones were used.
Here is a nice picture of some bits from an old Phone.
Modern STD dialling codes often match the letter codes on these old phones.
You have to ignore the 01 in the dialling code because this was added fairly recently. Also smaller towns use the dialling code of the nearest larger town.
Here is a big list of dialling codes if you are sad enough to see if this is really true.
So in the olden days, if you wanted to phone someone in Great Yarmouth, you would dial GY xxx. The GY translates into 49. Add the modern 01 prefix to get 0149. There is now a 3 on the end. I assume this last digit is to identify other places with GY/49 codes.
This does not work for all modern codes but most small and medium sized towns have kept their historical dialling codes.
This all goes to prove that there is nothing new under the sun.
Even mobile phone style abbreviations were used by Morse code operators over 100 years ago.
TTFN - Neil.