A security flaw in FaceTime allows you to secretly listen to an iPhone owner’s conversations without their knowledge.
Apple confirmed the bug. You better believe they’re working around the clock. They expect a software update sometime before Friday. In the meantime, they say they have disabled "group chat" in FaceTime.
The trick to spying on your iPhone friends is to make a FaceTime video call. While it's still ringing, swipe up from the bottom of the screen and click “Add Person.” Select your own phone number.
You will be able to hear the microphone on the device of the person you called. Whether they take your FaceTime call or not. (Variety)