I am also looking for an answer to this. For some reason for the past day or so when I am in a call, it will go to the lock screen (where you have to draw a pattern to open phone). When it goes to the lock screen I am able to hear the person on the other end, but that person can not hear me. If I look at the screen when this happens it will be on the lock screen and at the bottom it will say return to call in progress. It does this sometimes shortly after I am on a call and when it does they can not hear me unless I press the return to call button. So I went to the call settings and noticed the Voice Privacy setting and now I am also curious about what the setting is. Hopefully someone will have the answer to our question.