If you are involved in an incident but don't have a phone signal, if you dial 999 (or 112) this has the advantage that it will go via any available network not just the one you are subscribed to, it will also prioritise the call if networks are busy. So if your phone shows no signal, dial it anyway.
The disadvantage is that if it goes via another network, the emergency services can not call you back so you may need to call back to keep them updated from time to time.
The Essex and Hertfordshire Air ambulance has a phone app, that automatically finds your location and calls the emergency services at the push of a single button. I don't think it is restricted to Essex and Herts as the location facility is working fine and i'm currently in Northampton (Warwickshire & Northamptonshire Air Ambulance territory)
Search for EHAAT on the app store.
There are systems like the
SPOT tracker that use satellite technology but they are not cheap and require a subscription and from my own experience in Morocco, burn through batteries at an alarming rate!