Voice messages appear as unread until they are accessed over the phone or are marked as read in your e-mail client. The message waiting indicator (MWI) light on your desk phone remains switched on as long as one or more voice messages in your Messaging Inbox are marked as Unread. The MWI light is turned off when all voice messages in your Inbox are marked as Read.
Note If Outlook is in cached mode when you delete a message, the MWI light switches off only after the next synchronization between Outlook and Exchange .The MWI light is switched off when there is any change in Outlook that influences MWI, such as read or unread status change, deletion of a message, or moving a message to another folder.