Yes, pressing 0 Transfers to an extension, 99 in your case, and since 99 is outside the ACS's valid range of 10-57, you MUST set the phone status as "without a phone".
So, since the Partner Messaging knows that 99 does not have a phone associated with it, it doesn't try to send the call to an actual extension, but rather sends it to mailbox 99.
Since mailbox 99 points to Auto Attendant No. 1, that's what answers the transfer.
Now do this:
Create mailbox 55, WITH a phone, and point it to Auto Attendant No. 4
Go to Ext. 10 and program this:
Feature 00
Left Intercom Twice
#310-54-1
#321-54-1
Feature 00
Now press Intercom and call Ext. 54. Auto Attendant No. 4 will answer.
In your GUI, spend some time making different options for different selector codes. Send them to submenus, announcements, extensions, mailboxes, etc.
Experience is the best teacher. Spend time woodshedding on it!