I carry a phone for maps and emergencies. GPS will work w/o cell service, but you need to have maps downloaded to determine where you are. You can download maps from Google Maps beforehand. I've not had an occasion where I needed to call for help (knock on wood).
If I was traveling in a region where cell service is spotty or non-existent, I would do like I did in the 80's, carry a map, a set of tools and spares. If I was in a tough spot, I guess I'd hoof it until I could find someone who could assist me.