Hi steve,
Assuming zero is defined as attd in the dialplan, you need to make the Listed directory number night destination the voicemail access code. This will forward dial zero calls plus any other attendant type calls into Audix. Assuming a 4 digit dialplan, this sends the digit string "0000" into voicemail. In the audix programming you need to set up an auto attendant business schedule. You can probably use the default, then set up an auto attendant routing schedule. In that schedule you enter 0000 and set the destination to the mailbox number you wish. This basically translates 0 to a mailbox number.
If you can't follow this send me an e-mail to paul.beddows@telus.com, I'll send you some excel implementation sheets with an example. We do this all the time to given an attendant a mailbox.