There are 32 routing type mailboxes on the system. Any of these can be changed to a Directory that can be set to dial by either first or last name. From the Auto Attendant you would put in Goto XXX where XXX is the number from 001 to 032 of the mailbox you program to be a Directory mailbox.
Off the top of my head, you want to look at programs 47-07, 47-15, 47-13, 47-02, and make sure that names are programmed in 15-01-01.