The 803 12-Channel Programmable Master Station w/ 4-Wire Listen Option & IFB-4001 Emulate from Telex is the hub for the two-wire intercom system. Call signaling is standard on all twelve intercom channels. For applications requiring four-wire operation, IFB panel emulation or ISO panel emulation the circuitry is already there; you simply install an option cable to the back panel, set one or two internal DIP switches (IFB and ISO emulate only) and you’re ready to go. For all other applications, everything you need is in the box.