Once the digital changeover has happened, what happens if you need to dial 999 and find that your electricity is off?
If:
you normally have a strong mobile phone signal (not via Wifi Calling) in every major room of your house or business
the mobile phone mast(s) serving you still have power despite the outage (many will go off once grid power fails)
you have kept your mobile phone well charged
you have kept your mobile phone with you as you have moved around the house
... then there is no problem. You can call 999 from your mobile. And the emergency services will be able to call you back if they need to talk to you again (eg if an ambulance can't find your house).
If you have kept your phone charged, and you have it on you, the masts are still working, but are in a room where there isn't an adequate mobile signal on your normal network, you may still be able to call 999 using one of the other networks - for a 999 call, your phone will "roam" to any other network with an adequate signal. When your phone says "limited service", then this will probably work. (I was wrongly told by a BT executive that 999 calls will always get through, wherever you are: not true. Emergency calls do get priority if the network is congested, but if your phone is somewhere without a usable signal on any of the networks, you won't get through even on 999). Sadly, there seems no "999" coverage map, so you can't even get an approximate idea of how likely you are to get indoor 999 coverage.
If you can get through to 999 only via another network, will the emergency services be able to call you back (eg to clarify location if an ambulance can't find you)?
The answer is No. When I first asked, Ofcom couldn't tell me the answer: it took two letters via my MP before their Director General eventually confirmed this. Does Ofcom think that those who can only reach 999 via a different network should qualify for a Battery Backup unit (or other magical "solution" to the no-power no-mobile problem)? They're not telling - they just talk about people with "insufficient mobile coverage".
Wouldn't it be a good idea to let people check out whether they would be able to get through to 999 from their mobile - before they have an emergency?
I suggested to Ofcom that we needed a "998" service - routed the same way as 999 but terminated on an answering machine that would confirm that you would have got through had you dialled 999. That way, everybody could test the actual conditions to see whether the mobile network will manage a 999 call from their particular handset, in their particular location (eg in the bedroom, where you want to call if there is an emergency in the night). And they could test when it is raining or foggy (when mobile phone signals transmit less well) and they could test in summer (when trees have leaves, which can reduce signal transmission).
Ofcom noted my suggestion, but will make no substantive response.
So how can one check? My thanks to journalist Matthew Parris, who has looked into this, and has discovered that you can make advanced arrangements to make a test 999 call - you send an email (at least 48 hours ahead) saying what number you will call from and when (within a two hour window) you will call - instructions here. Hardly easy - we really do need something much simpler.