Apple recently fixed an issue with Group FaceTime that allowed users to hear a recipient's audio before they even picked up the phone, but it seems that the fix has somewhat hobbled the service.
Group facetime RSS
Apple is finally releasing iOS 12.1.4 today, which includes a fix for the FaceTime bug that allowed users to hear audio from people that they called, even if they hadn't answered the call yet.
Apple has issued an apology for the FaceTime bug which allowed you to eavesdrop on others during group calls. An update to squash the bug will be rolled out to the public next week.
A bug discovered in FaceTime allowed users to actually hear the person they were calling before they picked up the phone. Apple has since taken Group FaceTime calling offline for now.