@lthlth reading through the thread here, I can’t help but think that something has gotten lost in translation. As I understand it the ‘emergency backup line’ as it is often referred to as, is only available if the customer is registered as being ‘vulnerable’. So your patient would have been given one and this should allow them to make outgoing emergency calls (999, 111 etc.) over the 4G mobile network in the event of a power failure or internet outage. But I believe it is strictly limited to making outbound calls only and then only to a limited set of ‘emergency numbers’ so in the event of them having an internet outage, then I don’t believe they would be able to call your number anyway!
Also in your case, even if you were eligible for a backup line, I don’t believe that it allows for incoming calls anyway so in the event of you having an internet outage then you wouldn’t receive a call anyway. If as you say you only have four minutes to get to them, then I assume that you are physically very close, in which case there is a high chance that a power failure or outage is likely to effect both of you!
Now I am the first to admit that as I am not a VM telephony user, I am not entirely au-fait with the details of how it all works, but I have the bad feeling that you and your patient may have to rethink the arrangements once the 21CV migration happens to you both.
Is it possible that you can test it? ie if you were to go round to their house, leaving someone at yours and turned off their hub (hence simulating a power failure), can they still make an emergency call to you number and does it ring? If so, can the person at your house call them? If the answers to either of these are ‘no’ then I really think you both need to give this some considerable thought.