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During any audio/video call, by pressing your phone's volume up or Volume down key (depending on your preference), you can mute or un-mute your phone's microphone with ease.
Begin your audio or video call as usual. While listening you can press the volume +/- to mute your microphone. Meanwhile you can go to any other apps or browse the internet or even lock your phone and put it in your pocket. When it is time for you to talk, just press the volume +/- again and instantly your microphone will be un-muted and you can speak.