I'm not sure if it would be possible for you to consider using something like dialpad or skype VOIP
Dialpads pricing is quite competitive £12 a month for unlimited calling with lots of options is quite good.
Ultimately if you have anyone in your community who is vulnerable they need to tell openreach as in the latest UK Digital Event which they hosted recently they said that they want to know about it. They said that in those cases they would supply a UPS for the ONT however if you use a VOIP service they would still need to purchase UPS's which can cost anywhere from £50-£200 for a average user depending on what is connected and the load expected.
I use Skype VOIP as its free with my subscription to Office 365 as I get something like 80% off yearly however as you stated if they can use wifi calling this would be the best option but they would need a UPS for the router and ONT to ensure that they can call.
The other options are that residents take a look at
OFCOM coverage checker which is impartial to all providers and gives accurate coverage, I can confirm this as I have sim cards from all providers and the data they give matches what i actually get where i am.
However when i did some reading it is possible to link skype to a VOIP phone however when considering the costs its not worth it and a provider like BT or even going for talktalk and using wifi calling would work out cheaper in the long run as you don't need to pay for two phone systems.
I am going to tell you that personally if i get a choice of cheaper VM with a phone line or Openreach FTTP I will go to FTTP and just use WiFi calling as I get bad coverage as concrete walls and 3/4G does not go well together.
I also thought about getting a phone sim for an old iphone or something like that to just leave permanently at the house something like lebara mobile (note I'm not aware if WiFi calling is available) as it costs something like £3 a month with current deals so seems more viable.
Its ultimately your decision what to tell them but I would invest in a UPS take a look at ofcom checker to see what's available considering USwitch sim prices and if those sims have WiFI calling then that would be what i would go for as it just seems like the best option but i'll let the others tell you what they think but those are my thoughts.