[SR-Users] Kamailio Proxy for Freeswitch

So im using Kamailio as a load balancer and proxy in-front of a few Freeswitch boxes. Im not going to use RTP Proxy, as my FS boxes will be public on the internet ( Direct RTP connection is fine ). I want ALL SIP to go via Kamailio, but the issue im running into is that the SIP Contact header has the FS IP address in it in some cases. ( 180's etc ) Every bit of documentation I can find about re-writing the Contact header is geared around NAT, and thats not what im trying to do here.. Can anyone give some pointers on how to basically topology hide the FS boxes on the SIP front but leave their IP in the SDP. im going to have the firewalls setup so the SIP Can ONLY come from kamailio, so its quite important I get this all right. Any pointers would be appreciated. Thanks Jay