ITSPs
http://list.sipfoundry.org/archive/sipx-users/msg16333.html
Firewalls
http://list.sipfoundry.org/archive/sipx-users/msg16377.html
http://list.sipfoundry.org/archive/sipx-users/msg15468.html
http://sipxecs.blogspot.com/2009/08/sip-trunking-gotcha-with-pfsense.html
Configuration Tips
http://blog.myitdepartment.net/?p=52
(Thanks to Tony Garziano)