That's going to be a question for the CallManager and voice gateway folks and it will likely be dependent upon how the Cat is connected to the PSTN. Unity doesn't really know what its external number to the PTSN is. Typically the PBX is configured to route calls ringing in on external lines to specific phones (or in this case, voice mail ports). It's not configured from the actual end device, whether it be voice mail or a phone, it's done from the PBX.
Steve Olivier
Software Engineer
Cisco Systems