I'm trying to connect to asterisk which belongs to vendor, the vendor says that my invite got rejected because codec is not PCMA/PCMU.
as well as i need to set RTC port to 10K-40K to hear voice, while i plan to use RTCEngine later do i had to use it now?, since local call would still work without it.
so, can kamailio automatically negotiate and choose codec, (and probably RTC) on its own?. since i was expecting that the sngrep didn't give every info i need to know, and the rejection was caused by something else. i believe they used asterisk 20.xx
Kamailio do not negotiate any codec and not deal with SDP part(which have info about codecs) at all.
Kamailio do support ANY possible and not possible codec, just because it not deal with RTP.
You can deal with RTP using rtpproxy and "negotiate" codecs using SDPOPS module. Yes, you have add that code in your script, it is not in default script.
https://www.kamailio.org/docs/modules/devel/modules/sdpops.html
ps if you are using asterisk is waaay much simpler just allow asterisk do that.