Abstract: A headset or hand-free operating mode selecting and switching system for telephone is provided. The system mainly includes a circuit for a main unit of the telephone into which a headset connection detecting system and a single hand-free/headset button switch are added. By depressing the one single hand-free/headset button switch, the headset connection detecting system is automatically enabled to detect whether a headset is connected to the circuit of the main unit of the telephone. The telephone is automatically set to either the headset or the hand-free operating mode depending on a detected result from the headset connection detecting system. And, depressing of the one single hand-free/headset button switch again automatically disables the previously enabled hand-free or headset mode.