[SR-Users] early media: How to change to tag

HI I am trying to do the early media like this: PSTN/GW1 -> Kamailio -> Asterisk --> PSTN/GW2 GW1 send invite to Kamilio, Kamilio proxy to Asterisk, Asterisk reply with 183 ( with to tag=A) and sdp After the playing some medium, asterisk send 603 reply to Kamailio Kamilio intercept the 603, and lookup the routing table, decide to send invite to GW2. GW2 reply with 180/200OK with to tag=B In order to properly proxy the msg to GW1, Kamailio seems need to change the to tag from B to A. Is it the right way to do? If so how to change the to tag? or do I need B2BUA ( only signalling) to do the early media? thanks. min