The other portability issue we have here is where a company other than BT is providing the local loop, e.g. cable service in the larger towns. When this was starting out, changing to cable service meant getting a new number even if you were in the same house, because the prefix was used to route the call. With number portability, that's no longer the case. While it might be true that if you order a regular BT landline you'll get, say, a 40 prefix and if you order cable service you'll get something different, you can now port that existing 40 BT number over to cable, or vice versa.
May no longer be the case, but some phone companies tie the prefixes to certain towns. I used to live in Morrisville, where '-295-' and '-736- are mostly used.
The prefixes were tied to the central office, and many small towns had just the one exchange.
Go back 50+ years and many small towns in America had only 4-digit local numbers, and even then only a few of the thousands ranges might have been in use (e.g. 2xxx thru 6xxx).
When local numbers were made up to 7 digits for DDD, the office in that town just got the one prefix, and that's all it needed until such time (if any) as it outgrew the allocation. In fact in SxS offices in some places you could still make local calls within town by dialing just the last 4 or 5 digits (due to digit absorption in the selectors).
The same sort of thing happened within specific districts of larger cities. To take one example from over here, the Muswell Hill/East Finchley area of north London was served by the
TUDor (883) prefix. When numbers started to run short in the 1950s the GPO allocated a second prefix
HIGhgate Wood (444) which was located within the same central office building.
So somebody ordering new service in a particular house might have been given a
HIGhgate Wood number, but somebody moving into that house from anywhere else within that same area could most likely keep his existing
TUDor number if he wished.
By the way, for anyone interested in this sort of history I have a full list (PDF) of the old London director codes up until to the change to All-Figure Numbering in the 1960s. Just PM me for a copy.
