Mobile telephone. Mobile or cellular telephones use low-power radio signals to transmit to an antenna in the user's local area, or cell. The antenna is linked via telephone lines to an exchange devoted to mobile telephone calls. The exchange will connect the call to other mobile telephones, or if necessary to a normal telephone exchange if the call is not for another mobile telephone. If the mobile telephone moves out of its cell during a call, the exchange will automat- ically connect via the antenna of the new host cell. It does this by monitoring the change in the power of the radio signal between the mobile telephone and the antennae of adjacent cells.