Yes, But it has to be enabled first, so if you havn't enabled it previously I think you out of luck. But to be able to do it in the future In Unity go to the Class Of Service you have assigned for your users. Go to the Messages sub-page and select "Deleted Messaged are copied to the Deleted items folder".
Then when someone needs to undelete a message, you run a program called "Cisco Unity Message Store Manager" This can be obtained from CiscoUnityTools.com. You would run a script against the selected subscriber account to move messages from the deleted items folder back into the inbox.
Hope this helps.
If you do enable this, make sure you schedule a task in the Message Store Manager to delete messages from the dleted items folder. I have mine scheduled to run every eveing and it deletes messages older than 7 days.