An audio accessory key detection system (40) includes a host circuit (2-3) coupled to communicate via a microphone line (7) with an accessory circuit (3-3) in either a MSFT mode or a digital communication mode. Depletion mode transistors (44-1,2,3) in the accessory circuit are coupled between keys (15-1,2,3) of the accessory circuit, respectively. The depletion mode transistors are allowed to remain conductive for MSFT mode operation. For digital communications mode operation, the host circuit sends a command via the microphone line to a key detector and controller circuit (29A) in the accessory circuit. In response, a voltage is generated to turn the depletion mode transistors off so as to allow digital communications mode operation between the accessory circuit and the host circuit.