Unity auto-attendant functionality is not affected with a Legacy PBX integration. However, depending upon the type of PBX and the PBX's support of a voice mail integration, features may be limited.
It's all going to depend on the model of PBX. Some do great analog integrations (Avaya), some do cruddy analog integrations (Mitel). Some don't even do analog integrations (Nortel).
Steve Olivier
Software Engineer
Cisco Systems