The MWI (message waiting indicator) is activated whenever the mailbox receives a message that meets the criteria specified in the user's message waiting indication options.
The MWI DN is the mailbox user's extension which indicates that a message is waiting.
The MWI depends on the user interface:
On a digital phoneset, the message waiting indicator lights up.
On an analog phone the dial tone may be stuttered.
On the desktop, the message waiting indicator is an icon in the form of a red phone.