The IAX2 channel driver (chan_iax2) in Asterisk 1.2 before revision 72630 and 1.4 before revision 65679, when configured to allow unauthenticated calls, sends "early audio" to an unverified source IP address of a NEW message, which allows remote attackers to cause a denial of service (traffic amplification) via a spoofed NEW message.
https://exchange.xforce.ibmcloud.com/vulnerabilities/42049
http://www.altsci.com/concepts/page.php?s=asteri&p=1