First I went to the draytel.org site to check the settings from their help pages. The bit I had missed was adding a proxy server. Added that in a presto, registered straight away.
So what of the Telrad ? Well, getting help from them is like getting blood from a stone. However, as a matter of last resort I can speak to our engineer who is a hero. He's not really a mechanic - more of a fitter. Unfortunately, for quite a lot of things to do with our exchange, I know more about it than him.
Our particular exchange first got some form of VOIP upgrade but it was a bespoke thing and you you could only use a Telrad extension (though I also managed to hook it to an Asterisk box via a Trunk). Subsequently it has been fitted with a generic SIP card so any old softphone will now work. Goes pretty well via my Draytek router, though I do have the occasional router crash, particularly on incoming calls. Hopefully the firmware upgrade will sort that, but I don't have much hope !
Anyway, the upshot is that I added in the IP of the server as a proxy server as well, and blow me, it registered straight away. Not quite sure about why - I have two networks VPN'd over two routers. Must be to do with NAT between the two routers.
So make sure you set things as follows :
Domain/realm 10.0.x.x (IP address of the Telrad or SIP server)
Proxy As above (if there is NAT around)
Account number / Name : Extn number
Authentication ID ticked : Extn number
Password : your password
For Draytel, the first two are draytel.org
Just wish I knew how to sort out Hold/Transfer functions for Telrad off the phone. It's just a standard handset plugged into the router. I'd love to be able to suss this out so that I can pass calls back rather than saying 'they're busy - I'll get them to call you back'.
One of these days when I have nothing better to do !