A number of years ago I found myself in a new role responsible for consulting on the security of a VoIP platform.  Having never been responsible for VoIP security I went looking for Internet sources which I could use as a reference.  What I discovered was the Cisco SAFE security guides.  Although the SAFE VoIP security guide is long gone, the Cisco SAFE Security Reference Guide has recently been updated.  The Reference Guide is the omnibus document containing design standards for all aspects of network security.  Although Cisco-centric the Guide is a very complete and can be adapted to whatever technologies you choose to deploy in your networks.  In addition to the Reference Guide, the SAFE home page contains other papers which may be of use in interpreting SAFE.

-- Rick Wanner rwanner at isc dot sans dot org