Setting up the HT503 should actually be a lot easier than an SPA3102. Assuming the HT503 is on the same network as the Axon computer, you should be able to simply fill in the 3 details below in thr HT503 configuration page after creating an extension for the device in Axon's configuration page (assuming you are using it as an FXS device)
1. SIP server and/or Outbound Proxy FQDN or IP address
Set to the IP address of the Axon computer
2. User ID, Authentication ID
Set to username / extension number in Axon (User and Authentication should be the same)
3. Password
Set to password in extension
Save the configuration and reboot.