A person can open a cell phone, and if there is a signal, place a call to virtually any phone in the world. A person can be walking down a busy street where hundreds of others are using mobile phones and receive a phone call of their own from virtually anywhere. How exactly does that work?
Cellular telephones use radio waves to communicate with a cellular telephone tower which contains a transmittor/receiver. Why are they called cellular? Geographic areas are divided into cells, which can range in size from one half mile to 50 miles. Each cell has it's own tower which provides service for that area. The tower is then connected to a Cellular Telephone Switching Office (CTSO) or Mobile Telecommunications Switching Office (MTSO). The Switching Office is connected to the local telephone system.
Radio waves operate at different frequencies, and there are only a certain amount of frequencies available for specific applications. In each cell there is at least one channel, called the setup channel, which the cell telephone uses to transmit it's ID information. The cellular telephone company accepts the ID information and identifies the telephone and which cell it is in. The phone retransmits this ID information on the setup channel periodically to identify which cell it is located, in case the phone is moving around.
So how do you place a call? When a call is placed, the cell telephone transmits the dialed number and ID information on the setup channel, which is sent to the Switching Office. If the account is good, the Switching Office assigns a channel to be used. The cell telephone then uses the assigned channel for the call.
How do you receive a call? When there is an incoming call, the Switching Office sends the cell telephones ID information to the cell it is in, and the telephone seizes the setup channel and transmits it's ID information to the Switching Office. The Switching Office verifies the ID, then assigns a channel and connects the call.