ITSPs

http://list.sipfoundry.org/archive/sipx-users/msg16333.html

voip.ms how-to for newbies (assumes firewall and sipx system in place) http://myitdepartment.net/blog/191

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 Graziano)