Mr Red wrote: ...he reckoned the BT change over to digital is accompanied with the facility to be able to subtend extra numbers at the end of the dial sequence...

Adding codes, such as extension numbers, has been possible since STD was introduced into the UK in 1959. Remember the new phone boxes that took threepenny bits? It uses multifrequency dialling, which added * and # to the keypad on business, and later domestic, phones to send codes to PABX and answering machines.

STD also introduced the numeric area codes mentioned in the first post, so HARROW became 427 instead of HAR. But area codes haven't reliably worked for 30 years since BT lost the monopoly of issuing phone numbers. Remember Mercury Communications? There are several places, mainly rapidly-growing towns, where the area code system has been abandoned and 'local' numbers don't work; Milton Keynes is one.

I think the raison-d'être for digital (VOIP) voice lines is to allow video to be added - to match Skype etc. Also to abandon and stop maintaining the old copper (and Aluminium) wiring. It'll take longer than 2025 I think!