First of all, check if SIP Traffic is going through your SIP Server, not directly between Endpoints. To do this check if Record-route parameter is set in configuration file:

record_route();

Also, be sure that in SDP body you have External IP of your Kamailio/RTPproxy. If Kamailio is behind NAT you may need to use fix_nated_sdp function from nathelper module:

Regards,