Hi Guys,
I understand that Asterisk server behind a NAT and SIP pone behind another NAT will only work with use of STUN server.
Another option is site to site VPN, I use draytek routers which also have hood QOS capability.
A further option is to locate asterisk servers at each location and use IAX2 trunk between them.
Using SIP in double NAT scenario without STUN is not possible. (AIUI).
Good luck,
Sean Browne
Cardiff IT Support Ltd
0800 011 2931
www.cardiffitsupport.com