For additional information on Cisco IOS voice, consider the following books:

• Troubleshooting Cisco IP Telephony by Paul Giralt, Addis Hallmark, and Anne Smith. ISBN 1587050757.
• Voice over IP Fundamentals by Jonathan Davidson, James Peters, and Brian Gracely. ISBN 1578701686.
• Integrating Voice and Data Networks by Scott Keagy. ISBN 1578701961.