two possibles. 1 you are gripping the camera key. 2 proximity sensor isn't working and so screen is still activated against your face.
does phoned beep if you are using headset?
I have the same problem,when making calls i hear very loud beeping.I can't hear the people talking when the call connects,i can't hear the ring tone above the loud beeping sound.
Turned my phone off yesterday for 15 minutes so far today its ok,i did notice i seem to have a diffrent screen when the beeping happens to the screen i have when it works ok.Not sure if i am trying to make a video call by mistake when the problem occurs.I have only had the phone 1 week so still getting used to it.