During a call, press Mute button. The Message Waiting Indicator turns red and a microphone on the screen 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.