Create a new Call Handler
In the greeting, say something like " To search our Dallas Directory, press 1, to search our San Francisco, press 2, etc"
Create new Directory Handlers for each location.
For each site, create a new Public Distribution list and add everyone for that site in that list.
associate the Directory Handler to the Public List and it should work.
See this link:
https://www.cisco.com/en/US/docs/voice_ip_comm/unity/402/administration/guide/ex/SAG_0210.html