C-123LW: Dual master set Set includes a door station (C-D) and a 2 master stations (C-ML/A). Additional masters may be added. Each master requires a separate power supply, either the SKK-620 or 4 "C" batteries. The door station calls the masters by pressing a call button that initiates a chime. Communication is is Push to talk/release to listen at the master and hands free at the door station. Communication times out after being idle for twenty seconds.