I never tried because I rarely get any phone call now. But can you click (and hold) that small window, and drag it to the right? (off the screen)?
However, if you can really "remove" that window, the next question will be "how can I end the phone call"
It wont move, ending the phone call is simple you only have to click "End". This window is so obtrusive.
I mean if you can "remove" this windows, how to end the call? May be this is the reason why it must stay there.
AFAIK, when using FaceTime Audio if you get called, you can turn on Do Not Disturb and the window will disappear. If you call someone you can only remove the window by going into activity monitor, searching for FaceTimeNotification or something (don't know the exact name) and force quitting it. The window will disappear and the only way to stop the call is by going to the FaceTime icon in the Dock and quitting FaceTime.
Hope this helps.