Can't hear call and screen goes blank samsung s4

Hi, I was hoping to get a little help here, I recently replaced the glass on a s4, all went successful but now when in a call the other person can not hear me, only when I put it on loudspeaker they can hear me. Also when I unlock the screen 2 secs later it goes blank again.

From my experience with iPhones this would suggest its a faulty proximity sensor? What do you guys think?

The phone had taken a really bad hit to the screen prior to changing which could have caused this problem.

Any help greatful.

Thanks