If you press the "Message" key on the phone, the display should show more detail on where the MWI came from. If there's no message on the display at all, try using the Message Waiting - Deactivate feature access code (FAC + DN of the set), and see if that helps. If neither of these help, issue the Maintenance Command "locate feature extension <DN>" and see if the MWI is a Callback Message or Dialed Message Waiting.