https://www.theregister.co.uk/2019/01/29/facetime_bug/

In brief You might want to disable FaceTime on your iPhone, iPad, or Mac until Apple patches this bonkers bug.

Folks have confirmed it is possible to call someone via FaceTime, and secretly listen in on their device's microphone before they accept or reject a call. It's a handy, creepy way to find out what someone's up to before they answer. We're told iOS 12.1 and 12.2, and macOS Mojave are vulnerable at least.