The mute switch assembly screws should be tightened with the switch in the off position. Check the protrusion of the switch when turned on, since it may not stick out far enough to operate if the switch assembly is incorrectly positioned. (Check the gap between the switch frame and the bezel (white gap showing between the two screws near the 6). The screw on the far right is slightly longer than the other 3 screws. When reassembling the phone, keep this in mind.