https://www.cnet.com/news/apple-facetime-bug-lets-you-hear-iphone-mac-audio-before-the-call-is-connected/

We managed to demonstrate the bug in the CNET offices during a regular FaceTime call. After starting a FaceTime call with another iPhone user, swipe up from the bottom of the screen to add another user to the call and add your own phone number. While the phone is still ringing, you'll be able to hear audio from the recipient's phone, even though they haven't accepted the call. 

The Verge also spotted that if the recipient rejects the call by pressing the power button, video will also be broadcast from their phone. CNET recreated this, getting a second or two of video from the recipient's phone before the call was disconnected.