Absolute chaos
This seems a total mess.
Ofcom say that "providers" must provide battery backup for those without a mobile signal. BT seem to be ducking this by delaying the changeover for those who tell them that they have no mobile signal.
At least with BT, it is clear that they are on the hook for backup, eventually.
If you have a different ISP, who is the "provider"? My ISP, Plusnet (part of BT as it happens) says they are not going to provide voice services after the change, so have no obligation to provide a backup unit. The one VOIP company I know says they don't consider themselves as a "provider" in this context either. So hey presto - nobody is responsible for keeping no-mobile users able to make emergency calls when the power goes off. (Who is right? I've asked Ofcom, who have gone away to think about it, and then gone very quiet!)
Three more thoughts:
It isn't just whole-community power cuts to worry about. If your house catches fire, or suffers a flood, your consumer unit may well trip off. And without a backup unit, you are stuck if you live somewhere without an adequate mobile signal.
And beware the promises that you can call 999 using networks other than your own. Yes - you can call 999. But it seems that they can't call you back if you only have signal via a different network - so if 30 mins after your call, an ambulance is drifting around your village trying to find your address, they can't call you to ask for directions.
As to Backup Units, Ofcom confusingly talk about being able to make "an hour of calls" but of course what they mean is that the units will offer the chance to make calls during the first hour of a power outage. If your power goes off at 0100, then some time after 0200 the battery will be exhausted, so even if your 0500 attempt to call 999 is the first call you try, it will be too late. Who chose an hour? When there were concerns about the UK having insufficient generating capacity this last winter, the plan was for a rotation of three-hour power cuts.
It feels very much as if Ofcom has suffered from "regulatory capture" - they seem much better at meeting the needs of the telcos than at protecting the consumers who rely upon them.