As 9to5Mac points out, there’s a serious bug with FaceTime that allows callers to listen in on you regardless if you answer the FaceTime call or not. In some cases, callers can even see you through the phone’s camera.
This bug is specific to iOS devices running 12.1.x. Until Apple releases an update (hopefully later this week) it’s advised that you disable FaceTime for the time being. Apple has already taken Group FaceTime offline to partially mitigate the issue until they squash the bug that’s causing this. Expect an update from them soon.