As VoIP security threat is now an issue tackled in the industry, it is appropriate to take the precautionary measures to avoid attacks.
A web seminar is up online delivering topics concerning VoIP Security which is presented by Stuart McLeod. It covers all issues on VoIP threats and attacks, VoIP wiretapping, packet sniffing, infrastructure attacks and many others. The presentation is roughly around 21 minutes. This is brought us by Global Knowledge.
You can view the webinar at this link.

Nortel and SecureLogix had a strategic business partnership delivering a more secured VoIP services and management solution to corporate customers.
Nortel delivering communication systems to businesses, made this deal with SecureLogix which is a key player in enterprise telephony management with its ETM System.
SecureLogix’s ETM System is a comprehensive voice network security and management platform, enabling customers to protect critical voice and data network resources from telephony-based attack and abuse, optimize their telecom resources and expenditures, and simplify voice network management. PBX independent, ETM is fully functional in TDM and VoIP/UC environments.
[ via FierceVoIP ]

According to a research article I read, VoIP calls can carry secret messages using stenography. This can be noticed with some fuzzy sounds during your VoIP calls; it may not be noticed by those who are not equipped with necessary devices.
Messages can be encoded in the bit of string which when decoded represents some important information about something. With this, hackers and terrorists can use it as an tool in transmitting information with their colleagues.
To understand more about this revealing research, you can follow the links below:
http://www.vnnforum.com/showthread.php?t=73731
Secret messages could be hidden in net phone calls