Is there a way to force a busy no answer if I do not have a phone built?

Example:

IPCX, customer hits a button to transfer to voicemail. Hits CUCM and a line that is built for this. Line is configured for Call Forward all, which goes to Unity and plays the standard greeting. Our goal is to play the busy no answer greeting. But kind of hard to do if I cant make the call busy on CUCM, then to unity. In Unity, I do not want to activate the Busy greeting as primary for other reason.

Am I stuck? Or is there a way on CUCM to classify that call as Busy, then to the Unity subscriber?