I wonder how I can show
FaceTimeNotificationCenterService again if I run the command
killall -kill FaceTimeNotificationCenterServicebefore?
For the next call, the notification should appear again. But I don't think you can get the window back for the current call as the notification is gone.
If your MacBook sports a touch bar, you can end a call, switch to video or muting using it. It's not possible to dial numbers though ("Select 1 for department A, Select 2 for department B").
Don't forget to provide Apple feedback on this. Even though this issue is around for a few years, hopefully they'll find a better solution than the current notification. https://www.apple.com/feedback/macbook.html