Spare sim from another company might help, orange/t mobile do all right for west brecons. Smart phones can give a gps reference and baring which is usefull if you can get an emergency signal so help can easily locate a position
I am with orange/T mobile, nowadays it is called EE. Thing is when you are in the shadow of the mountains, you can't get anything, you have to be sufficiently high up or near somewhere urban in my experience.
I have lost my track higher up on Snowdon too, when the mist comes down well it is not hard to wander off, not so far as to get lost but to find the footing difficult of the main path. Saw someone slip up there too, could have been nasty but fortunately it wasn't.