Partner Messaging or Partner Voice Messaging?
Partner Messaging (the module) out of the box has NO password set for the system administrator's mailbox, and the minimum password length is set to 6 digits. The first person to access the system administrator's mailbox gets to set the password. There were 3 revisions, 1, 6, and 7, they each have a different backdoor password.
Partner Voice Messaging (the PCMCIA card) has 4 digit passwords, and the typical sequential one is assigned to the System Administrator's mailbox. There IS a backdoor, but it can only be used locally, not remotely.