[SR-Users] Client Registration and Call over Kamailio

Hi all, I am using Kamailio as a proxy in front of a 3rd part Open IMS (Not IMS modules in kamailio). I have defined Kamailio as an outbound proxy in Soft clients(I have configured IMS users info in Soft Clients). When I send register from client to IMS, Register message is sent to IMS over Kamailio. Kamailio forwards this received Register to the 3rd party IMS. Finally the client is registered into IMS over Kamailio. However when I try to make a call from registered users (both registered over Kamailio), IMS sends 403 Forbidden - "Not Registered! You must register first with a S-CSCF" to Kamailio. When I debugged the issue I found the reason why IMS sends 403. When IMS receives an Invite message from Kamailio it checks whether source IP which Invite is sent registered or not. As I make registration over Kamailio without changing contact header, IMS registers the user with the contact IP (which is the client IP). So to make a call from Soft client (IMS users) over Kamailio I also need to register Kamailio source IP as well.To deal with the issue I have changed the contact address of Register message coming from Soft client with Kamailio IP on Kamailio. Then IMS registered Kamailio IP as a contact when registering IMS user. But this time when I make a call from Soft client over Kamailio, IMS sends Termination Invite to Kamailio. So to finalize the call Kamailio has to route this termination Invite to the terminating IMS user . Is there any better approach to overcome this issue. Which is the better approach to use Kamailio in front of IMS. I want to both register and make a call from Soft clients over Kamailio to the IMS. Regards, Serkan CELIK