I have an extension w/three VMS keys and a sub with their mailbox numer. Two of the keys indicate with a MWI led but one does not. The VXC has all three mailboxes set up the same and I tried a different key on the existing phone w/no change. There's another phone in the same facility set up w/three VMS keys & different subs set with their mailbox numbers and it works fine.
What am I missing

Be sure the mailbox has MWI set in voicemail D screen. If the mailbox was set as "phantom" (i.e. no extension number populated), voicemail will never try and activate MWI.
Mike