TDoS: The latest wave of Denial of Service attacks
by Corey Nachreiner - Director of Security Strategy at WatchGuard - Monday, 15 April 2013.
That said, some VoIP TDoS attacks can appear as perfectly legitimate calls—because they are. They just come from a malicious source. Differentiating these calls from legitimate ones can be challenging, even with a hardened VoIP server and the right protections.
If you suffer a TDoS attack, like one ones recently reported, contact the authorities, and also contact your VoIP service provider to see if they can help upstream.