If you use the Mitel Headset designed for the 53xx phones, when the call rings the user hears a beep in the headset and simply presses a button on the headset to answer.
Also, you can use Plantronics headsets that uses a lifter. The phone rings, the user hears a beep in the headset, presses a button on the headset, a mechanical lifter picks up the handset and the call is answered.
Ralph