A critical bug has been discovered in Apple FaceTime that transmits audio(in certain cases video) to the caller despite the recipient not having accepted the call. This vulnerability was first discovered by a 14-year-old kid who found that he could listen to his friend's conversation, even when the call wasn't answered. This incident was tried hard to bring into Apple's attention since Jan 20 by the kid's mom but was in vain. On Monday, Jan 28 this incident was finally brought to Apple's focus by 9to5mac.
Apple's action:
Until Apple releases a fix for this bug, Desktop Central's workaround can be handy in avoiding other users from eavesdropping you.
Follow this document to block FaceTime by executing custom scripts on Mac machines.
Cheers!