Lock screen during call to disable mute FaceTime

Your proximity sensor should turn the screen off. Do you have a screen protector or something blocking it?

Jailbreak get Calllock it auto locks screen to prevent the proximity sensor issue.

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.

Just lock the screen with the sleep/wake button. It will bring you to the lock screen at first, but the screen will then go off, or you can force it to actually go off when you press sleep/wake again.