Dave posted while I was typing, but I’ll post it anyway…
You would assign a Call Routing Announcement (CRA), with an appropriate greeting, as the Announcement and Overflow stations in the hunt group.
An Announcement is played once. An Overflow is played each time the Overflow timer expires. They can both be the same CRA, or different ones. In programming, go to hunt groups and press F1 for help. It’s explained pretty well.
The Time Out destination on the CRA needs to be Hang Up. Anything else will pull the call out of queue.
As to your unused numbers, you could point them to a CRA that plays a few seconds of busy signal and drops the call. Low tech, but it works. Just keep the recording short, as you’re using a voice mail port for the duration of the call.