According to NIST, with the proliferation of VOIP, the demands for security are significantly compounded. Now, network administrators must protect two invaluable assets - our data and our conversations. Federal agencies are required by law to protect a great deal of information, even if it is unclassified. The current Internet architecture does not provide the same physical wire security as the phone lines. What's the solution? Encryption! Encryption! Encryption!
Encrypting VOIP traffic and running it over a virtual private network provides excellent security when dealing with external communications. Architecture decision, like locating IP Telephones behind NATs and Firewalls, are also important.