When you make a call on your mobile phone, it emits electromagnetic radio waves also known as radio frequency, or RF energy. Antenna from the nearest cell phone tower will then receive these radio waves.
Cell phone tower consists of antennas that both transmit and receive signals from mobile phones.
After receiving signal from a mobile phone, the cell phone tower then transmits the signals to a "switching center" - a telephone exchange for mobile phones. Here the call is connected either to another mobile phone or to telephone network.