Re: 100% 999 coverage?
There are plenty of places in the UK, some quite near to major urban areas, where the mobile signal is non-existent for all networks!
Brailles, Warwickshire. Only twenty minutes from Banbury. I'm with Vodafone and there is no signal in parts of the village or at a nearby golf course. I can't even send a text. My mate is with another network and he says the same thing. We've often thought that it makes that golf course a bit more dangerous than most in the area because if you have a heart attack while playing someone will have to get back to the club house (which could take fifteen minutes) to raise the alarm.