https://www.theverge.com/2019/1/29/18201931/how-to-turn-off-facetime-iphone-ipad-mac

Want to listen in on your friend’s conversations? Thanks to a bug in iOS 12.1, all you need to do is call them via FaceTime. You can then use their phone’s microphone to hear what’s going on, even if they don’t pick up the call.

This serious bug is now known by anyone who reads tech news. (It has been tested and verified by The Verge staff.) It’s actually quite easy to prevent anyone else from invading your conversational privacy until Apple is able to fix the bug later this week. Just turn FaceTime off. This will disable both video and audio FaceTime calls, but it’s a safety precaution to take until this nasty bug is fixed.