You do not need to disable System Integrity Protection (SIP).
To delete the FaceTime app, do the following:
Backup FaceTime.app via Time Machine or making a Zip archive.
- For the Zip archive, right-click it and select: Compress "FaceTime"
Shutdown your Mac.
Boot to Recovery Mode. Press ⌘R when starting your Mac.
Open Terminal from the Utilities menu.
Run the following compound command adjusting Macintosh HD as necessary if you've modified the default name of the primary startup volume:
rm -r "/Volumes/Macintosh HD/Applications/FaceTime.app"; reboot
After rebooting from Recovery Mode, if you still have the FaceTime icon on the Dock, right-click it and select: Options > Remove from Dock
Note that while this does remove the FaceTime.app, it may/will reappear when macOS is updated/upgraded and will need to be deleted again.
If after having done this and you want to restore the FaceTime app, then restore it using Time Machine, if you backed it up in that manner, or if you created the Zip archive, it would have been created in the /Applications folder, if you moved it elsewhere, move it back and then do the following:
Shutdown your Mac.
Boot to Recovery Mode. Press ⌘R when starting your Mac.
Open Terminal from the Utilities menu.
Run the following compound command adjusting Macintosh HD as necessary if you've modified the default name of the primary startup volume:
cd "/Volumes/Macintosh HD/Applications"; ../usr/bin/unzip FaceTime.zip; reboot