Thanks for the replies.
Current set-up is: BT £12.09 LR, £13.52 unlimited (Total £25.61); Age 91, memory issues (not recalling they have just called someone) so unlimited was good (in the past, regularly over 3000 mins per month - but current calls are less than 10 mins per month but could change); 3x DECT with Trucall Call Blocker/Screener. No catchup. No mobile - tried a Doro 7010 in 2021 but couldn't get along with it. There's a stairlift and house alarm but these are not connected to the NTE (afaik). Many powercuts (can be 3 or 4 times a year), although MiL lives in an urban area - and although MiL has PSTN it wouldn't work during the cuts due to DECT base not having power; problems with noisy line / cannot make calls over the last few years. One of the daughters is two doors down, and good neighbours.
I hadn't realised PDPL was for edge cases, for those with other legacy equipment and from the above, MiL is not an edge case.
FTTP is available and I could look around for (a shorter) 12m contract. MiL may need to move residence so wouldn't want a long contract because ETCs would be payable. Could look at FTTP for around £25 p/m and then VOIP with A&A. This would be about the same as current BT cost - assuming calls stay at 10 min/month. If calls increase then Voipfone do an unlimited for £20 per user per month - but now looking at £45 p/m compared with the existing £25 p/m. Probably use a HT801V2 (newer and better interface than the V1) connected to the existing TruCall base for familiarity.
If only MiL could get along with a mobile. We could just then more to VOIP and re-direct incoming calls to the mobile.
Perhaps one of the neighbours can provide a network cable to which the HT801V2 could be connected?
aquiss FTTP 80/20 and A&A VOIP