I actually had the same problem, what I do is after the call is connected push the top button (the one that makes the screen go blank) and you can't touch any buttons that way. At first it was annoying but after about 20 calls it worked out well.
Btw: I was on the phone with my wife (been married a month yesterday) she asked what I was doing for her and I had forgotten to do this and the phone hung up on her.. Lol.. Good times until no sexy time that night
