[SR-Users] Kamailio Load balancing multiple asterisk boxes

Hi I have two boxes with internal and external interfaces. Each has Kamailio at the public interface and asterisk on the internal one. Making calls within a box is fine but would like to load balance using the other asterisk box. However using dispatcher I have noticed packets are not being routed from Kamailo to the second asterisk. Packets from Kamailo are originating from the external interface and sent to the second asterisk box using private ip address. Off course this is not routable. I have tried masquerading but no joy. Do I need to change kamailio configuration? I want to have a set of asterisk boxes behind Kamailio using dispatcher. I'm I missing a trick? Eric