"A significant bug has been discovered in FaceTime and is currently spreading virally over social media. The bug lets you call anyone with FaceTime, and immediately hear the audio coming from their phone — before the person on the other end has accepted or rejected the incoming call. Apple says the issue will be addressed in a software update 'later this week'."
"Naturally, this poses a pretty privacy problem as you can essentially listen in on any iOS user, although it still rings like normal, so you can’t be 100% covert about it. Nevertheless, there is no indication on the recipient’s side that you could hear any of their audio."