Hi, After asking many questions, I haven't got any clues about how Kamailio handles INVITE message by default, in terms of modifying c= line in SDP According to rtpproxy flow http://kamailio.org/docs/ser-getting-started/SER-GettingStarted.pdf When client register, SIP proxy will call nat_uac_test() to detected if client is NATed or not, then save this info. When client A calls client B, the INVITE message will go through SIP proxy. Here the SIP proxy can do 3 things (as in section "INVITEs behind NAT" in the pdf). 1. Add an SDP command direction:active to the SDP content 2. Change the c= line to a.b.c.d 3. Force RTP to go through a proxy by changing the c-line to c=IN IP4 address-of-proxy and the m-line to m=audio port-on-proxy RTP/AVP 0 101. When will SER do 2, 3 ? -- Khoa Pham HCMC University of Science Faculty of Information Technology