From a technical standpoint this is possible but I personally wouldn't touch it with a 10 foot poll unless you have serious bandwidth, very low latency and QoS between the Unity server and the Exchange server(s). This is not something Cisco has tested so no support would be implied.
If the Unity server could sit next to the Exchange server in the hosted facility and then connect over a point to point WAN link with QoS to the CallManager you would be running a supported configuration.
The AD schema will need to be extended as well. I could see the management company pushing back on that one.
Keith
Keith Chambers
Unity Technical Lead
Unified Voice Team, San Jose
Cisco Systems
