There are a number of reasons your cell phone will drop a call. One of the very basic reasons is that you are or have moved to far away from a cell tower. Tall buildings, mountains, thick foliage, and of course bad weather can all contribute to a call being dropped.
What about when I?m driving on a beautiful sunny day in the middle of the desert and my phone decides it's tired and drops the call. All of the major carriers are always looking to overlap their coverage areas, but unfortunately there is a limit to the networks architecture therefore you will sometimes run into an areas called ?dead spots?. Dead spots occur where there is no antenna available to continue to transmit the signal.(duh)
Another reason that's not so ?captain obvious? are that solar flares can contribute to a number of dropped calls particularly during sunrise and sunset. This is because when cell towers are erected they are positioned to face east and west to communicate across the country. A call can be dropped without warning even if you have full bars of signal.
Tips for limiting dropping calls:
