I have not had this problem, but I just had a thought that might be relevant, which is not that common! With this all-glass, capacitive, multi-touch screen, is it possible that part of your hand that is holding the phone is also touching the glass? I experimented with this today. I called my work number (knowing that it would just ring and allow me to experiment) and I made sure my left hand was touching the glass while it held the phone. When I was doing this, no matter how many times I pressed the end-call button with my right hand, it would not disconnect. As soon as I shifted the phone to where my left hand was only touching the case, the call ended immediately on the first touch.
Possible source of your problem?