Hi,
Nothing to my knowledge about RFC's around port selection, but best practice is to use high numbered ports in UDP to not cause any clashes.
If you use RTPengine with your Kamailio setup you can manage the port range via the rtpengine.conf file.
Specifically the options:
port-min = 10000
port-max = 20000
It's important to note that Kamailio doesn't touch RTP itself and the port selection is done by what is handling the RTP. So your carrier selects return port (your egress) and the media handler selects your ports (ingress)
For example - if I were using Kamailio as a proxy and used FreeSWITCH behind it, RTP port selection would be selected by the FreeSWITCH itself.
Thanks,
John.