Sure. When the user enters voice mail through the message button, it asks for a mailbox. The user would then enter whichever mailbox they are checking, whether it be their own extension or someone elses. Then when it asks for the password, they would have to enter the password for the mailbox that they are accessing.
So user yyyy would enter xxxx's mailbox (ext #) and xxxx's password from yyyy's phone.
Hope this helps!
Lgarza