Klaus Darilion published an interesting paper explaining what happened to German VoIP users and how to mitigate. I suggest that you read this one. Looks like attacks are becoming more and more widespread / mainstream.
Sandro Gauci, Enable Security
Analysis of a VoIP Attack
Published on Oct 24, 2008 in voip security, cyber crime
Sandro Gauci
CEO, Chief Mischief Officer at Enable Security
Sandro Gauci leads the operations and research at Enable Security. He is the original developer of SIPVicious OSS, the SIP security testing toolset. His role is to focus on the vision of the company, design offensive security tools and engage in security research and testing. Therefore, he is the proud owner of the title of Chief Mischief Officer at Enable Security.
He offers public office hours and is reachable here.