On the CTI RP, configure the BUSY, NO ANSWER and Failure to where you want the call to go. Don't forget the proper Calling Search space.

You MAY be able to check the VM box for the no answer, busy, failure if that's what you want to do in your design.

Remember that Unity (if you are thinking of using a Unity Auto Attendant) only accepts the Original Called party number. So Unity will get the DN assigned to the CTI RP. You will need to put that DN on the appropriate Unity object (subscriber or call handler.)