VoIP with SIP

[calinb]

Try http://www.diamondcard.us/ I used to call via them some time ago. May be they are still running. There is a service to get a number in virtually any country of the world
I personally prefer modern solutions aka Riot Web, etc. For instance because SIP has several design security bugs I wouldn't recommend it for personal use unless you are really an iptable pro

I'm far from an iptables pro but I've used shorewall firewall ("iptables made easy") and it worked like a charm for me on several occasions when I've had complex routing challenges. Perhaps it would mitigate this challenge?