Quick and Easy way...
1. Change the greeting on the DN to play the announcement you want the callers to hear. I'd also state in this greeting, "Stop this is an announcement, DO NOT LEAVE A MESSAGE in this mailbox".
2. On the phone itself, press the FORWARD key and forward all calls directly into vmail.
Using this method, when the position is ready to be used again, the DN & Mailbox is still setup.
The other way is to setup an Announcement.
This has to be done through Vmail Admin... a little steppy
If you decide to use the announcement method, I'll send you step-by-step instructions.