The guys on the transmission line project carried satellite phones (expensive international dialling) until we could get the cell companies to extend their coverage. Remote isolated and exposed working, with some working a long way from anyone else. Survival training, survival kit and regular reporting in. Only one twisted ankle that we learnt of, and his team got to him in less than twenty minutes, then got him back to the road before the ambulance arrived. Know where the guys are, be confident they know how to read a map. Supply epirb if you are really worried. And flares,smoke grenades (to help the heli guys land when necessary) and make sure people can give location info in a way the emergency services understand. There are NO 'apps' that can short cut safety in remote isolated working... Especially when the phone battery dies because its been hunting signal... Hope this helps.