During a call on your Polycom SoundPoint 650 phone, press the Mute button. The key lights up and a flashing microphone on the graphic display indicates that the other party (or parties) cannot hear you.
Microphone Mute applies to all modes: Handset, Headset, and Hands-free Speakerphone. You can still hear all other parties while mute is enabled.
To turn off Microphone Mute, press Mute button again.