Can you pull the card out to have a look at the label? There were 4 versions of voicemail made for the Partner systems.
9991 would be the General Mailbox for Automated Attendant 1 on a later Partner Mail, or a Partner Messaging. On Partner Mail, you could move the Administrator's Mailbox functions to any other mailbox, so if you were the in-house support, and you sat at Ext. 15, you could have your own mailbox also be the System Administrator's mailbox. All of the others are fixed.

Partner Mail, the external PC-on-the-wall, 9997# (initially)
Partner Mail VS, 99#
Partner Messaging, 0#
Partner Voice Messaging, 0#