Good question Dan. I agree, the IP gateways are great ways to extend existing PBX functionality.
When a call is sent over the IP Gateway to an endpoint, its my impression the PBX does it in a call forwarding type scenario. Meaning the PBX cannot detect if the endpoint is busy or RNA. Obviously many PBX’s can perform alternative call handling based upon busy or RNA such as rolling over the call to an alternative location(s), placing the caller in Q, etc. In a gateway scneraio, I believe I lose that ability for the PBX to make high level judgement calls about call handling as the PBX isn’t in complete control of the call.
That is at least my impression, but would love your feedback.