I don't think there is a native report. Back at my last company, we used Vitel Software's VMACS plus. It would poll the audix and run a variety of reports (including the activity log on all mailboxes) you select. It then would parse them down and present you with the ability to run reports against the stored data. Vitel doesn't make that package anymore, but I think they came out with a newer software package that performs the same functions.
If you poll all the activity logs, the process takes awhile. But one of the reports in the old package was the "Unused Mailbox" report - one of my favorites.
Just wish that Avaya would include better reporting in their pbx and voicemail systems. But then, other companies wouldn't have this nice little niche.