Logs are easy to find: open web interface (not the admin interface) as admin user, click DIAGNOSTICS tab and you'll see all sorts of logs. There's a VOICE MAIL section with AVDAP logs, if it's anywhere I'd expect it there. Much to my surprise I actually did find a record of bad login call when I purposely tried bad pass. Do a search for "Invalid password" and you should see the three attempts...scroll up a few lines and you'll find the record of what outside or internal number logged in.
If you haven't used web interface before browse to PBX IP address, enter admin login and password (same as you use for DB programming). Let us know the end of your story!