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Can we add option to set Ring Group for Desktop/mobile app and desk phone (ring simultaneously) then forward the call to a backup number after certain number of rings then forward to Voicemail if not answered. I know ring group and simultaneous ringing are available however I'm looking forward to exclude a number to ring at a later time.

Yes, go to users, then the user from the user list that you want this to apply to.  Click call handling and forwarding.

Activate the ways you want to receive calls.  Example:  Desktop and Mobile App, Desktop Phone, etc.  Place the desktop handset last.  Set the ones you want to ring above the desktop handset and select the "ring for" to 0 seconds.  It seems counterintuitive, but this is how I got it to ring everything simultaneously.