When you make a call on your wireless phone, your phone generates a radio signal. This signal is picked up by an antenna on the nearby cell site tower. This antenna is tuned to a specific frequency so it can tell the difference in a cellular phone, radio station, television station, etc. The cellular tower redirects the signal from your phone to a switching station which reads the contents of your phone's radio transmission. It sees the number you are trying to reach and it redirects your call along a specific line to its final destination. After the connection is established, your phone sends your voice as a radio signal to the tower, through the switch and to the final destination of the call. When you listen, you are hearing signals from the cellular tower received by your phone. Since radio signals weaken the farther away they travel, you must be within range of a cellular tower to make and receive calls. Any objects between your phone and the cell tower can cause interference in the signal. The range of each tower can be different.
The network automatically switches you from cell site to cell site. This is what will allow you to automatically receive or make calls.