I guess I am confused... if you have a separate network element, communicating with a different external FQDN/IP then how is the invite being sent to the wrong carrier/IP? You essentially treat each SIP element as a separate trunk group, then access it via ARS and try a call, the invite should go to the MBG which then adjusts the IP's for the network translation (but shouldn't change it's destination) and forwards it off on the public interface to the destination carrier...