On the Extension page for each user who need to “monitor” this voice mailbox, add the extension # for the mailbox they need to monitor, separated by a comma.
Example:
For extension 101, I have this entry:
mailbox: 101@default,400@default
This user will now be monitoring mailboxes 101 and 400, and the message waiting light will activate if there is an unheard message in either mailbox.