I have been using a VFX line connected to an Asterisk server for several months with no problems. The Asterisk server has just failed so as a temporary measure I configured my VoIP Gateway - an ATCOM AG-188N - to connect directly to VFX so I can use the phone while it is fixed.
Everything appears OK, the gateway says that it is registered and I can make calls successfully, the problem is that I can only receive calls for about 60 seconds after the registration. After this time incoming calls go straight to the World Exchange voice mail.
If I connect to the gateway control page using a browser and cause a re-registration (by hitting APPLY on the config page) I can again receive calls for about 60 seconds.
The gateway always says that it is registered, I called WxC and they said that the device was showing as registered even though I could not receive calls at the time.
My gateway is connected to Telstra broadband (static IP) via an Apple Airport Extreme, the gateway also has a fixed IP. The hardware configuration has not changed since I moved from Asterisk to a direct registration. The Asterisk server was located remotely, not within my network.
The SIP config is as follows:

Any suggestions about how I can receive calls would be great, is there a difference in the configuration needed between connection via Asterisk and a direct registration from my device ? - when the VFX line was set up I specified that the connection was via Asterisk.
Thanks
Bob