A bug discovered with Group FaceTime allows a caller to listen to your Microphone before you answer the call.
The bug was reported to Apple a week before it hit the media. Once it hit the Media Apple shutdown the Group FaceTime service to limit possible privacy violations.
Group FaceTime was released last year as part of iOS 12 and allows up to 32 participants to participate in a group call.
Apple has identified a fix for the issue and has started working on the fix for the issue. It will be releasing iOS 12.1.4 later within the next week.