IAX is a native protocol, used by the Asterisk open-source telephony platform. It has been superceded by the SIP protocol but is still supported and used extensively.
IAX uses a single User Datagram Protocol (UDP) data stream between endpoints for both the session signalling and the media payloads, it uses the port number 4569. This feature provides benefits for traversing network address translators on network boundaries, as it simplifies firewall configuration.
Further information about IAX is available here.