CG-NAT seems to be problem with sip/VoIP. There is a thread here https://www.geekzone.co.nz/forums.asp?forumid=137&topicid=197892 They suggest using the 2talk sip proxy. It's ages since I fiddled with this, but the other thing was setting it up to use a STUN server - there is info under the 2talk website about that. I seem to recall it has the. Downside of making calls out take quite a few seconds to get going.
Perhaps you could check out if skinny can provide you with a static or public in address. (sorry, just found on another thread they can't/don't offer either of those services)
Nothing is impossible for the man who doesn't have to do it himself - A. H. Weiler
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