Some alarming reports of a serious error in Group FaceTime associated with iOS 12.1 or later, have emerged. This bug allows an iPhone user to activate the call microphone of a second iPhone, whether its own user is aware of this or not. Apple has announced that they are working on a solution, and has also addressed it in the interim by shutting the video-call service down on their end.
https://www.notebookcheck.net/Group-Face-gate-New-iOS-bug-grants-access-to-iPhone-mic-audio-without-its-owner-s-knowledge.401576.0.html