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.
Set for First Time Enrollment. Either 0 or 1. 0 means the user is no longer flagged for first time enrollment, a 1 means they are. Typically an administrator will set everyone up for first time enrollment and when they call in and go through the enrollment this flag is set to 0. This can be a handy way of telling who has not yet setup their mailbox so admins can go by and gently remind users they need to do this.
"Every fool's got a reason to feelin' sorry for himself" - Springsteen