The problem in most of the cases are the same... NAT Symetric, that is used in most of the modem-routers broke the communications. Sometimes after a few seconds, sometimes in one way communication only, sometimes with SIP registration but no RTP success.
NAT IS NOT COMPATIBLE WITH VOIP.
There are thousands of documents about this problem, with this virtual PBX and all others.
You just googling: NAT and SIP and be ready to stay maaany hours reading possible expensive and complicated solutions.
Theorical solutions:
ALG (don't try it)
Universal Plug 'n Play (don't like it)
Direct connection to Internet (IP must stand on the Axon computer, not on the router) (Theoricaly it can works, but ther are negative points asswell)
And dedicated hardware.
In my case, the finest solution is to get a modem-router with DMZ host feature. It's extremly easy to configure.
When you try to do anything, like transfer a call or record a call with a VRS server... etc. NAT don't know how to send the udp packets to one IP : port to another IP : port.
DMZ sends all the IP / UDP packet lost of destination to an assigned computer on the lan. (Axon computer) Making possible the communication without any problem.
Salute