You can digitally network the Unity servers to create dialing domains. When forwarding a voicemail to ext 8888, unity will pop the question.... "Press 1 to send to John doe at 8888 or press 2 for jane doe at 8888"
To make this work, you have to digitally network Unity together. But it also depends on some other topology plans.... if you are using CallManager.. how are using Callmanager... Centralized betweeen the sites? Distrbuted?