Voice over Internet Protocol (VoIP) networks have freed users from the tyranny of big telecom, allowing people to make phone calls over the Internet at very low or no cost. But while VoIP is easy and cheap, it's notoriously lacking in security. With minimal effort, hackers can eavesdrop on conversations, disrupt phone calls, change caller IDs, insert unwanted audio into existing phone calls, and access sensitive information.

Hacking VoIP takes a dual approach to VoIP security, explaining its many security holes to hackers and administrators. If you're serious about security, and...