Key System – A business phone system that allows users to access more than one telephone line and more than one line from more than one telephone, and place a line on hold in order to answer or initiate calls on other lines. It is usually include an intercom capability that allows users on different telephones in the system to communicate with one another. The number of station and phone lines served by a key telephone system is limited by the size of the key service unit. On a typical key telephone system, each incoming phone line appears on a separate button on every key telephone set, which is typically a small incandescent lamp or light-emitting diode and lets the user tell if the line is in use. The same lamp flashes at a certain rate when an incoming call is ringing on the line, and flashes at a different rate when a call is on hold.