As some of the previous posts said, I have only seen that once and it was when my phone was sitting in my car in the summer. I have never had that happen during a call, and again...it only happened once. I left my phone accidentally in my car while I ran somewhere. It was sitting with the windows up and in the sun, so it makes sense....your situation, however, seems a little odd to me. I would set up a genius bar appointment, explain the situation, and see what they say. That is what I would do at lease if this occured while I was just making a phone call.