How can I mute the microphone in a phone call? Simple question, but there seems to be no option to mute the microphone in the call app.. or am I missing something?

(I have tried 3rd party apps, but without success - will fire up the Android Studio if there are no answers to this question!)
What version of Android are you using? 

Usually the option to mute the microphone is available directly from the call screen.



It would be useful a screenshot of your call app.