The document discusses securing VoIP networks with open source tools. It describes how open source testing tools like SiVuS and SIP Bomber can be used to assess the security of VoIP implementations. SiVuS is a vulnerability scanner that discovers SIP components, generates attack messages, and produces a security findings report. SIP Bomber tests SIP protocol implementations by generating messages and validating passwords. The document advocates using open source tools to securely configure servers, clients, gateways, and firewalls before deploying VoIP networks.