If you were to turn on the system wide messaging archive Messaging->Messaging Management> and check the box next to system wide message archive, you could achieve what you are looking for.
You'll need to provide an email address to send all the messages to, but this will provide a copy of every message the system receives from every mailbox. I archive to an exchange mailbox that I set up specifically for this purpose. vmarchive@xxxxxxxxxxx.xxx - I keep 1 week live in the mailbox, and the last 30 days in an exchange archive file; plus I still have my backups if I need to go back further than that. You can search for user name, mailbox number, time, date, whatever...
The messages are compact in Nortel's proprietary .vbk format, however you will need storage capacity on your email server to accommodate the number and size of the messages. My 30 day file runs about 2-5 gig worth of space...
We crank through somewhere between 400-900 voicemails a day in rough numbers....
Good Luck,
30n30w