The TCB-2 automatically answers a telephone line on the first ring and disconnects that line when the calling party hangs up. It is ideal for unattended situations such as listen lines, IFB feeds, dial-up network, remote transmitter sites and satellite links. The TCB-2 needs a momentary open or reversal from the central office to disconnect. Most but not all CO's provide this signal. The TCB-2 is powered by an external AC or DC Supply.