...changed his assigned voice mail password...
Not to be a smart-a**, but if you don't know the user password it needs to be reset in a manner similiar to how it was assigned in the first place (most good password systems will not allow you to view the current password). That being said, if you have no-one inhouse that knows how to set up new mailboxes or edit existing ones, you need to call in the vendor that set the system up for you.
I currently support two DK40s and a DK280; the DK280 has a ContactDK voicemail card in one of the expansion cabinets, one of the DK40s has a notebook-sized box on the wall next to the main system, and the other uses a PC computer for it's voicemail. Each of these systems is administered differently, and none of them use the 4 digits followed by a #. But again, unless you or somone in your organization knows the password to administer the voicemail box/card it really becomes a moot point to figure out exactly which one you are using.