Mostly what you do is you figure where half the distance to the next tower is, then make some adjustments for terrain.
Out in the boonies, you're usually only in contact with one cell site. In a more urban sitiation, the system computer is constantly making a comparison of signal strength from whatever repeaters you happen to be hitting. When any one of them has a stronger signal, your call gets switched over to that site.
Dropped calls are usually because NO site has a signal from you.
Thanks for the additional info.