Cell towers do not work individually, each tower only has a few
channels of the huge amout of frequencies available. Your cell phone
simply picks the strongest available frequency. That is usually the
nearest cell tower, but if all available channels are in use that may
be the next nearest tower. There are not just cells but also regions
of cell towers, each region will use all of the available channels and
have a limited number of land lines available to it. The problem
occures when the cell phone access several regions with several towers
of the same frequency. Aircell phones have a lower power and a shaped
antenna giving a norrow beam down to the ground.