There are a number of ways an end-user may learn that they have new voice mail, such as from client software like Microsoft Outlook, OWA, Lync, Communicator, or different types of indicators on IP, smart and traditional phones. The term Message Waiting Indicator (MWI) is usually applied to notifications delivered by the user’s phone using mechanisms such as a light on the phone, a special icon or highlighted notification or even a special dial tone. Exchange 2010 includes built-in support for MWI.
For more details, see Understanding Message Waiting Indicator in Exchange 2010 documentation.
Exchange 2010 Unified Messaging also includes a feature called Voice Mail Preview which uses automatic speech recognition (ASR) to add a text version of the voice mail that’s delivered to the user’s Inbox (along with the MP3 voice message). This allows users to read their voicemail messages. See Voice Mail Preview Advisor for Exchange 2010 for details.
Read more at source: http://blogs.technet.com/b/exchange/archive/2011/06/13/troubleshooting-the-messaging-waiting-indicator-status-in-exchange-2010-unified-messaging-server.aspx