It sounds like there would be a double NAT involved here at the very least.
ISP <-> NAT <-> CLIENT <-> NAT <-> VOIPo
@cfabe
If you have an example Call-ID (You can grab this Call Reference ID by clicking on Call From or Call To of the applicable call from Call History). I can further look into this and provide some possible insight as to the problem.
However with this kind of a NAT setup it is likely going to be a simple issue of blocked packets / traffic.
Bookmarks