Make Facetime ignore incoming calls in macOS
Have you tried disabling notifications? Or at least turning them down?

Under  → System Preferences → Notifications → FaceTime, uncheck Play sound for notifications, then change the Alert Style to Banners. Banners shouldn’t steal focus.



You could also enable Do Not Disturb while in fullscreen by ⌥ opt-clicking the Notification Center icon at the far right of your system menu bar. Some apps do this automatically, while others observe the When mirroring to TVs and projectors setting under the Turn on Do Not Disturb header.

This is accessed by going into  → System Preferences → Notifications → Do Not Disturb.