Hi Guys,
Sadly every year ClueCon overlaps with my travel for defcon/blackhat but this year there was a fairly interesting tool released found at
http://viproy.com/ for metasploit. It allows for a very large and robust set of VOIP attacks from DOS and fuzzing to some interesting manipulation support.
It is not a point and go tool to get the most benefit but can certainly stress a variety of things on a VOIP server.
I haven't had a chance to do too many fuzzing and depth attacks although on an old FS server I had it was vulnerable to an invite attack (most likely due to a configuration error on my part, although I didn't change my acl settings much on the updated server so I am not sure).
If you use TLS only with certificate validation though you are secure against just about any sort of attack:)