Hi Simon,
You need to use the public IP address that the Axon computer is connected on, in the 'Server (SIP Proxy or Virtual PBX)' section of express talk.
Though Express Talk probably still won't be able to see the Axon Computer because the Network Address Translation on the internet router would most likely be blocking all the ports to that computer.
What you would need to do is forward ports 5060-5065, 5070-5075, 8000-8010, and 606 if using on hold music, from the Axon computer by using the settings in the router to do this.
And Yes this does work.