Craig,
Do you have an internal partition only? An internal only calling search space? One way would be to create a partition for the intercom, then make sure that no other partitions except the internal partitions can reach it, that will exclude any inbound numbers. My internal partition in Internal_PT under my calling search spaces I have also called it Internal_PTCSS. Then under the calling search space configuration page, only select the internal partitions that you want to be able to dial the intercom. Do not give that calling search space access to all available partitions. You could also do it under the Intercom calling search space and partition under CCMAdmin GUI Call Routing >>> Intercom. There is some additional functionality to the intercom piece, that you will want to read up on before you do it that way though.
