Once I have upgraded to Digital Voice, and the phone socket stops working, what happens if I sell my property? Does the new buyer then only have Digital Voice as an option, and then have to use broadband to get a line? Or can they get a new line set up in the old way?
Their new provider would be responsible for that. All providers are moving to some form of digital phone system. The copper connection will no longer be used to carry the phone service, only broadband.
Digital Voice is BT’s proprietary VoIP system , no one is obliged to use BT, so it’s only if the new occupant of your property chooses BT as their provider will they have to use DV if they want a ‘landline’ phone service, they could of course chose BT and have FTTP broadband without any phone service at all , it’s not compulsory.
As far as choosing a company that will use legacy PSTN , if there is a stop sale of copper products on that exchange , then it may be impossible to order a legacy connection, and by 2025 all exchanges will be in that category , as far as LLU MPF providers , you would need to ask them.