The Message Store Manager will kick out all kinds of detailed information about the mailbox state for users includng how many unread voice mail messages they have.
You can, of course, dump the MWI status from the Unity DB but you're wanting to get the mail count directly from the Exchange mailstore, right? So you would need to use MSM or a similiar tool (Microsoft has the Mailbox Manager which also has a reporting mode).
You can check out MSM on its home page here:
http://www.ciscounitytools.com/App_MSM.htm