https://motherboard.vice.com/en_us/article/3kgqvn/facetime-apple-bug-how-to-turn-off

A new creepy flaw in Apple’s popular video and audio call app FaceTime allows the caller to listen in on the receiver’s iPhone, even if that person has not picked up the call, according to multiple news reports and users who claims they have experienced the bug.

The bug allows someone who calls another person to hear the audio from the iPhone or Mac of the person they’re calling by adding yourself to the call. Here’s how 9 to 5 Mac, one of the first publications to report and reproduce the bug, explained it: