The Auto Attendant runs on Cisco Unity Express (CUE) on the UC500. Cisco Configuration Assistant is used to configure the AA, which includes Open and Closed greetings, menu prompts, Prompt management extension, prompt management authorized users, and in the AA_SBCS_v03.aef script (comes packaged in the 8.0.4 UC500 SW Pack) a no action transfer destination.
Partners have discovered that the Prompt Management menu includes the capability to record an “alternate greeting”, which plays as a prefix to open or closed greeting (depending on the schedule set in CCA for business hours).
This doc shows how to make it work