There is an app called "Find my friends" in the Play Store, download it into the hidden phone and your smart phone, it uses GPS to locate you and your "friend" and displays the location on a map and it is fairly accurate. Granted both phones still need cell service, but as far as a battery backup, a hidden car phone charger hard wired to a live car battery circuit and plugged into the "hidden" phone should keep that phone going for a very long time.