There does not appear to be any way to avoid getting these unwanted calls, short of turning off FaceTime. Luckily, it's easy to do.
How to Turn Off FaceTime on Your iPhone and iPad
- Open the Settings app.
- Scroll down to FaceTime.
- Click on the toggle next to "FaceTime."
How to Turn Off FaceTime on macOS
- Open up the FaceTime app.
- Click on "FaceTime" in the menu bar.
- Select "Turn FaceTime Off."
With FaceTime disabled, no one will be able to secretly FaceTime call you to listen to your audio. With FaceTime enabled, anyone can exploit a Group FaceTime bug to place a call to you that will allow them to listen to your audio even if you don't accept the FaceTime call.