Apple has activated group calls via FaceTime with iOS 12.1 on October 30. As its name suggests, this feature allows to make FaceTime video calls with several people. Now Apple has temporarily disabled this functionality due to a critical error discovered yesterday.
The error allows user to listen to another person by initiating a FaceTime call with the recipient, and adding as their third party their own phone number.
In this way, FaceTime considered it as a FaceTime group call and activated the microphone of the person you were calling before even answering the call. In fact, even if the recipient did not accept the call, their audio became part of that conversation.