Dear oh dear. With our 36-hour power outage and an over 3-day Internet outage, we relied on our POTS phone to keep in touch. But it's not so convenient to lose access to our distributed phone set-up (only a bunch of BT8610 cordless but it suits us).
So I think I'll look into VOIP. What I want is a base station to replace the one we have but it'll have an RJ45 instead of a phone socket on the back, and I'll connect it to an RJ45 socket on the router and Voila! away we go with VOIP. I'll repurpose an old UPS and since the base station and the router use about a microwatt each, it can power the router and the base station without mains power for about 1000 years so that at the next power outage, we can carry on using the phones all over the house exactly as before.
Now for the reality. Well, I can't even find out about the reality. VOIP is packetised voice over the Internet, obvs, but can I find out more? Unlikely. Winky gives me guff about protocols and standards, but little else. Google isn't much more help. Most info sems to refer to the US and its providers. I find some Siemens cordless listed which are "VOIP compatible" but that tells me nothing.
And don't anyone say that we don't have to worry about this yet. SWMBO's aunt moved into new-build in Yorkshire only this week. No landline and no provision for one, but there is FTTP. So she's reliant on the mobile for the moment, but suppose she wants a regular geographical number that people can call and which will cause all her cordless phones scattered over the house, to ring. What then?