I use VoIP services from 3 VSPs (2 in Australia and one in UK) in addition to my UK POTS line. I don't understand those saying voice quality on VoIP is worse than POTS what they have probably experienced is the 'VoIP' solution they get from the likes of WhatsAPP and SKYPE which don't emulate a true VoIP connection..
My understanding is that in a POTS call only the last leg (exchange to subscriber house) is true POTS all the intermediate switching and routing is VoIP. I'm pretty satisfied with my VoIP connections they sound better than the POTS, as there are no crackles and clicks and, as I run my own modest Raspberry Pi VoIP server, I benefit from lots of Value Added Features and the ability to manage my own least cost routing.
I only ask that in whatever final solution BT propose they don't lock it down to their own 'approved' equipment like some Australian ISPs have done with their National Broadband Network VoIP services.
The UK should adopt a standard VoIP solution AND customers should be allowed to have their login credentials so they can connect to the widest possible selection of terminal equipment. With the right thinking the solution could also provide a 'number for life' meaning when you move home you don't have to change your telephone number.
What we don't want is BT repeating the telephone socket fiasco, inventing a completely new connection plug just to frustrate people wanting to do their own house wiring. Most of the world manages reasonably well with the RJ series of connectors. (yes I understand the 'bell wire' ring detect capacitor and anti tinkle but that could have been handled in a RJ solution.)
The real challenges are provision of services at distant locations, where no internet reaches, and solutions for the probable thousands of legacy POTS installations for base alarms, emergency alerts emergency phones and the like. Education is essential as people will need to understand that the old way of providing extensions phones at home by plugging them in parallel won't work for VoIP unless the VoIP modem provides a POTS simulated connection.