Sometimes out in the middle of nowhere there can be a cell signal that is too weak to register on your phone but a booster will get you through.
A yagi antenna and a booster can sometimes help. This gentleman does a nice write up on boosters etc. Communication
Also, if you like to boondock in the middle of nowhere but want to let someone know your still alive, you can get a SPOT SATELLITE MESSENGER :: HOME PAGE
that uses the satellites.