Hi Sebastien,
I think a great place to start would be to use this excellent Unity Tool;
Cisco Unity Tools - Subscriber Information Dump (this can create a .csv file that contains much information including the following)
Date of subscriber creation. The date the subscriber was added to the Unity system.
Time of Last Call. Indicates the date and time the user called in to check messages last. This can be handy for finding out which boxes have not been access at all or which ones have not been accessed in a long time. This value is in GMT and not your local time, keep that in mind.
http://www.ciscounitytools.com/APP_subr_info_dump_3x.htm
This can also show mailboxes that were created but the "self-enrollment" was never completed.
From Jeff Lindborg;
"Information about messages (i.e. how many and which are marked new etc...) can be generated using the message store manager tool which you can get here:"
http://www.ciscounitytools.com/App_MSM.htm
Hope this helps!
Rob
