[Kamailio-Users] Processing ACK.

I am having an issue while setting up Kamailio. I using the carrierroute modules to process calls between carriers. The configuration works great. However I have and issue. On the SIP messages We are forcing all the media through an RTP proxy, however my issue has to do with the Contact field on the Message Header. While I am able to use force_rtp_proxy to replace the contact and connection fields on the SDP part of the message, I can seem to modify the Contact on the Header part. I tried using subst to change the Contact:, and while I was partly successful, it seems the ACKs going from one carrier to another are getting messed up and throwing Kamailio into a loop. We are trying to avoid providing Contact: information to our carriers. Basically keeping them from know about each other. Any ideas/help will be Greatly appreciated.