have you checked the Unity Event Viewer for any issues with this account? Usually Unity will flag a message if it can not deliver a message. Usually, its a permissions problem in AD, but if others have issue, its could be more.
Is this voicemail only? UM? Exchange 03 or 07?
If the person has existing voicemails in their mailbox, can you mark them unread then toggle to read and watch the MWI light go on/off? Can the person dial into the TUI and check old messages? If they can not, its a permissions issue with this account.