Two things: 1. You don't have to use your active phone as a your gps. For a whole host of reasons many folks are buying a second phone for short money- < $100 to use as a dedicated GPS. (ex. Kyocera DuraForce Pro). Your gps phone doesn't need to be active on a calling plan. 2. For the very reason you outline (no signal) it's probably a bad idea to depend on a cell phone for emergency rescue. Get a PLB, SPOT, or inReach for that and always wear it on your person.