Hi -
Not sure if you have already resolved this ... just saw the post and wanted to respond. If you move an unread voice message to the Deleted Items folder or other folder, it turns off MWI. Sounds like MWI is still ON for this user, but the TUI doesn't pick up the voicemail message. Are you seeing any errors in Unity's application event log for this user, for example to indicate Unity doesn't have the necessary permissions to access it? If you have resolved, I would be interested in what the problem was and how you addressed it.
Ginger