This appears to be a real issue that Apple is resolving this week!
Earlier today, there were discoveries of folks using iPhone's FaceTime app.
When the person being called doesn't answer the video chat, (you) could still access Without Permission that person's mic and audio! Yikes...
For the moment, as a Just in Case measure you could
Go to your Settings > FaceTime > (turn off or disable by tapping on toggle till it's no longer green)