Electric current is transmitted over wires to another telephone, and the diaphragm in that telephone vibrates in response to changes in the electric circuit's magnetic field. So when the person you're calling picks up the phone, he or she can hear your voice. Long-distance telephone calls can also use radio waves or microwaves. Microwaves can be sent to an orbiting communications satellite and relayed back to the earth. Microwaves are used in microwave ovens, of course, but in radar and communications links as well.
Computers can communicate over telephone circuits via a modem.
The cellular telephone doesn't need wires, so you can take it almost anywhere. A cellular phone is a small radio transmitter. The phone's signals are too weak to be heard at long distances. The phone sends and receives microwave signals to a base transmitter and receiving station. A cellular telephone is called that because a geographic region serving the cellular system is divided into areas called cells.