Cellular Phones used the land based towers to communicate. When we are in a particular area the coverage is provided by the tower close to us. As we move away from the tower we get poor coverage/signal or we can say the weak signal or call drop problem on our phones. The reason is that either in that area there is not any tower or the tower is too far from you.
While on the other hand, Satellite phones donot depend on the Towers, But instead transmit the signals by satellites orbiting the earth. Satellite Phones directly transmit to the satellite near to it which then sends the signal to nearest control segments, or land-based center, which then transmit the signal to the receiving phone, which can be land line phone, cell phone or another satellite phone. In emergency condition Satellite phones are more useful because they donot depend on Towers.
But if we come to cost factor the abundance of cell phone and easy availability of creating land-based networks makes the cell phones and calling by cell phones more affordable.