Whoa! If you have an Iphone and are on IOS 12.1 or later (including 12.2 beta), there is a very serious bug that allows a person that is FaceTiming you to hear you before you even answer!
According to MacRumors:
"This bug is making the rounds on social media, and as 9to5Mac points out, there are major privacy concerns involved. You can force a FaceTime call with someone and hear what they're saying, perhaps even without their knowledge."
Apple did say that they are aware of the bug and have already disabled Group FaceTime.
An update should be out later this week that will fix this bug