I don't think that's possible, once the call goes out from CUCM all the handling is done in the GW and CUCM has no idea of anything else related to where and how the call is routed in H323. You only get the H245 and H225 signaling related to establishing the call but none of that has the info of which carrier you're using.
HTH
java
if this helps, please rate
www.cisco.com/go/pdihelpdesk
HTH
java
if this helps, please rate