Mobile phones have what is called a baseband ASIC which is based on a certain technology (CDMA, WCDMA, LTE). These mobiles talk to base stations, large towers with antennas mounted on top. Both the base station and the mobile have send and receive capability. They both speak the same language. As the mobile phone gets farther away from teh base station, the base station asks the mobile phone to increase its power. As it gets close, the base station has the mobile decrease its power. As the mobile gets to the edge of the base stations cell coverage, through the mobile switching center (MSC) it alerts the neighboring base station that a mobile is approaching. The current base station then perform a handoff to the next base station and that happens as you travel from one cell site to the next. You keep getting handed off to the closet base station. It's all rather amazing what goes on behind the scenes when you make a simple cell phone call from yor car while driving.