You can set it so that no Face Time calls or alerts will be silenced. You can set it so Face Time calls from your “Favorites” are exempt from “Do Not Disturb”. You can also turn on “Repeated Calls” to allow an important Face Time call to come through if someone tries to Face Time you repeatedly. When you have “Do Not Disturb” turned ON, a sliver of a moon will appear next to the time at the top of your screen.