It depends how the unity server is intergrated to the PBX or CallManager. I think what you are trying to do is to have subscribers use the "cross logon" access correct?
Unity is able to log users in whatever system they are in if you have it setup correctly with digital networking, etc.
For Unity, I usually setup local route points for each office so they have a "local number" to access to Unity. Then I customize the call handler to tell them they have reached company xyz office voicemail system, to login, press blah blah blah.
hope this helps