The Asterisk development team has released new versions of Asterisk for 1.2.x, 1.4.x, 1.6.0.x and 1.6.1.x that correct an IAX2 denial of service vulnerability. The security advisory (AST-2009-006)  indicates that 220.127.116.11 and 18.104.22.168 correct the vulnerability.
This vulnerability affects all versions of Asterisk prior to 1.2.35, 22.214.171.124, 126.96.36.199, adn 188.8.131.52. Package updates for Fedora are required; apparently this changes the IAX2 protocol as well so I'm not sure if a backport is feasible.
MITRE's CVE-2009-2346 record:
The IAX2 protocol implementation in Asterisk Open Source 1.2.x before
1.2.35, 1.4.x before 184.108.40.206, 1.6.0.x before 220.127.116.11, and 1.6.1.x
before 18.104.22.168; Business Edition B.x.x before B.2.5.10, C.2.x before
C.2.4.3, and C.3.x before C.3.1.1; and s800i 1.3.x before 22.214.171.124
allows remote attackers to cause a denial of service (call-number
exhaustion) by initiating many IAX2 message exchanges, a related issue
asterisk-126.96.36.199-1.fc10 has been pushed to the Fedora 10 stable repository. If problems still persist, please make note of it in this bug report.
asterisk-188.8.131.52-1.fc11 has been pushed to the Fedora 11 stable repository. If problems still persist, please make note of it in this bug report.