Use the speakerphone
Short answer
Use your Emberton III as a speaker phone when connected. Push the control knob once to accept a call, or twice to reject. Push the Bluetooth button to use your phone instead.
The microphone lets you use Emberton III as a speaker phone:
Push the control knob once to answer or end a call.
Push the control knob twice to reject a call.
Push the Bluetooth button to toggle the call between the speaker and your phone.