Do you have the prox sensor on? For some reason mine was off by default when I got mine. Go into the phone app. Tap menu (three dots top right corner). Call settings. Scroll down to prox sensor and make sure it's checked. Scroll further down and check out the gesture options. I like the answer when bringing phone to face.
I think it's a little pinhole looking thing, on the top left side of the screen. Turns off the screen when talking so you don't accidentally hang up, or when you put an opened phone in your pocket.
I have mine set so call is answered when the phone is raised to my face. During the conversation, screen blanks but will auto switch back on (prox sensor) when away from my face in the event I need to enter password or access codes for automated phone menus etc.