I have a quite similar problem which I reported Monday to technical support but got no answer yet:
My PC has 2 IP addresses on the same network card. I can see in my network traces that Axon is always using "the second IP" as the IP address. Since this second IP address is in a public range and doesn't actually belong to my network, the SIP provider never finds my Axon.
I also think it's a bug since setting a static address in Axon doesn't help. It still uses the wrong IP Address.
(btw: I have this weird network configuration because I have a VoIP phone which tries to connect to some pre-programed provider which I "static-route" to my Axon instead)