Cheek - Mute
When I hold my conventional Android phone (from my first HTC Evo to my Google Nexus 6 to my current Essential PH1) up to my head in the ordinary way, I frequently have problems. Because the "Mute" button on the phone's touchscreen is positioned so that my cheek often touches the Mute button.
I know, the phone is SUPPOSED to blank the screen when it's next to my head - but it often takes a significant fraction of a second to do so, and during that period it's susceptible to "Cheek Mute".
That's one reason why I prefer using a headset, either BlueTooth or wired.