According to 9to5Mac and other social posts, Apple’s FaceTime software allows users to eavesdrop on a call even without answering it.
The bug is easy to replicate, dial another user on your iPhone. While that user is their phone, will need swipe up on the call screen then and add themselves via the “Add Person” screen before the call is answered. FaceTime then believes that a Group FaceTime conference call is occurring and transmits audio from the recipient’s phone without them ever accepting or rejecting the call.
Apple has identified the issue and plans to release a fix later this week.