NIST (National Institute of Standards and Technology) has a special publication, 800-58, released January 2005, entitled Security Considerations for Voice Over IP Systems. Go to the NIST publications page and search for 800-58. (Or go directly to the PDF.) The 93-page publication is a great resource for planning a new VoIP installation or reviewing security measures on an existing system. In addition to security recommendations, it has good technical overviews of major VoIP protocols (H.323, SIP, MGCP) and network design considerations.