I had this problem using a case that just clipped round the edge, it didn't even touch the front of the phone and yet I still kept muting people with my face.
never happened with a Bumper, which covered more of the front face of the iPhone, totally confused me, so now i just don't bother with a case, it looks much nicer without it anyway.
a way I found of getting round it while on a call, is to bring up the keypad, yes you may press a few numbers with your face, but at least it can't mute.