Assuming you have routers with firewalls on both side.
Axon Setup
Ensure your router is forwarding 5060 (Signal) and 8000-8499 (Audio) ports (TCP & UDP) to the internal IP of the server.
You need an external address for your server.
If you have a fixed IP you can use that, if not register with either No-IP or Dyndns.org to get a domain name that points to your internet IP address.
Now set up the extension you want to register.
Remote setup
Depending on your phone you now need to work out the signal port Audio Ports, for my Polycom IP500 this is 5065 and 4000-4049, you need to check on your specific model details.
In my example the remote router need to forward ports 5065 (Signal) and 4000-4049 (Audio) ports (TCP & UDP
The phone must have a fixed IP address on DCHP allocated. Now forward the ports 5065 (Signal) and 4000-4049 (Audio) to the IP of the phone.
Then register the phone to the extension number at the external address of the Axon router (either the IP or domain name you registered on port 5060
Final Set UP
Make sure the external IP address in Axon is set the external IP or domain name of your router and in extension set up tick “Always Relay Audio” to ensure the audio streams is forced between the server a remote phone not directly between the internal and external phone when the audio will probably fail with one way audio.