Most of the problems on Nokia E7-00 appear, when the user answers or makes a call holding the phone by the left ear and the proximity sensor somehow by-passes the ear. So the touch screen stays activated and the user accidentally presses hold call or end call button on the screen with the cheek.
The proximity sensor location near the E7-00 marking is closer to the left side of the phone, and when holding the phone by the left ear it is very easy for the sensor to fall on outside the ear area. When this happens the operation of the proximity sensor fails. The distance gets too long for the sensor to detect any presence of macroscopic object (skin, hair, fabric) so the touch interface and the display fail to close.
There have also been incidents where users have been accidentally using or playing with a lock key during a call, and by activating the lock key they have disabled the proximity sensor so it is not controlling the touch and the display anymore, and false presses could be possible. Improvements have been done in Symbian Anna and Nokia Belle.