There is also an Auto Attendant, which could answer first, offer the caller the option of dialing an extension number, and then transferring to that extension. If the extension is not answered, the extension's personal mailbox answers instead.
Or, lines could be answered directly by a mailbox, like a elaborate answering machine.
When you call into your office, not when you press Intercom 777 to check messages, what do you hear and what are your options.
If it is an auto attendant, then you need to log into mailbox 0, the system administrator, and follow the prompts to change the Auto Attendant prompt.
The password for the system administrator's mailbox will be found in the manual that came with the voice mail system (not the red fold-out user's guide)