I have constant issues with the iPhone 4s hanging up while talking. It seems the proximity sensor isn't working the way i thought it would. If I place the phone under my chin while talking, it seems that my face either "mutes" the call or many times "ends" it by hanging up. I am using a BookBook case (which I really like) but I wonder if this is causing my problem. Also, where exactly is the sensor? Anybody got any suggestions?