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