To reset (Initialize), thanks to http://www.tek-tips.com/faqs.cfm?fid=2754
Key hint is the "secondary" password: 654321 to input after dialing 9 for Administration Main Menu in order to Modify a Mailbox.
007 MLM Voicemail System admin:
1. Dail 770 (or voicemail) for mailbox system.
2. 9997# for admin
3. password: 123456 (if changed and forgotten,
use S/N: 211515 to initialized)
4. dial 9 to enter higher level admin mode
5. password: 654321
Now just follow the messages to modify any specific extention and choose Initialize password to reset it to 123456.