After learning about the basic VoIP protocols, it's a good idea to familiarize yourself with their security weaknesses....
H.323 and SIP, the two main protocols used by VoIP hardware, are both plagued with security issues that network consultants and systems integrators should be aware of when working with their customers. This tip examines some of these inherent weaknesses.
VoIP protocol insecurity
SIP and H.323 have both been plagued by security issues. SIP because of its simplicity and H.323 in spite of its broader design parameters. To be fair, many other Internet protocols are vulnerable to spoofing or buffer overflows. But given the high availability associated with the public switched telephone network, companies moving to VoIP may be more sensitive to these threats. Furthermore, as RFC 3261 acknowledges, "SIP is not an easy protocol to secure. Its use of intermediaries, its multi-faceted trust relationships, its expected usage between elements with no trust at all and its user-to-user operation make security far from trivial."
Learn more about the vulnerabilities of H.323 and SIP.
Get more information on VoIP protocols.