Asterisk outgoing calls through HT503

I’ve just bought a Grandstream HT503 and I can get it to route incoming calls via Asterisk but when I try to get an outgoing call through the FXO port Asterisk tells me the call is not permitted and the HT503 log shows Invalid User Id on the outgoing call but I have the SIP details set up so that even if I allow the HT503 to register with Asterisk the credentials all match.

Is there anywhere that there is a really good description of the Asterisk and HT503 settings to get them to work together?

My initial problem was solved by setting Stage Method to 1 (default is 2). Now the call actually gets out to the telephone line but I have a secondary problem that I get a message saying that the number called isn’t recognised (I’m in UK) so not sure what’s going wrong at that level.

I’ve managed to sort out the problem of dialing in UK. It seems that you need to set AC Termination model to Impedance based with a value of 600ohms. I now have my HT503 dialling out succesfully and it works well without echo.