Not taking on new users doesn't always mean end of service. Hopefully, anyway.

Also, FFS. It's always the simple stuff. Finally works after 5 hours of messing about. Just had to disable stun

Here we go, basic HOWTO - Thompson Router (585v8, ymmv) and Obi100 (ditto):
THOMPSON
1. Hard reset to default settings - button on the rear. If you've not messed anything up in telnet or haven't got tons of custom port forwards enabled, fine, but you've been warned

2. Thompson can stay at default settings (well, change the admin pass asap, and enter your isp details). By default, UPNP is enabled - it can stay like this. Nothing default actually needs changing other than...
3. The *only* change you need is to disable SIP ALG - telnet to the thompson, "connection unbind application=SIP port=5060", "saveall", "system reboot".
Without step 3, the Obi will never be able to register with Sipgate / SIP, irrespective of other settings. Soon as you hit enter on the first command, it pretty much registers immediately, but save and reboot anyway.
OBI
- Setup is as
https://secure.sipgate.co.uk/user/configreader.php?show_conf=Obitalk (basically all the stuff welshpaul put above)
- Do NOT enable the stun server. enabling will allow rings in, but pickup will have the caller keep ringing.
- For added bonus, I entered all the uk ring settings at
https://www.ukvoipforums.com/viewtopic.php?f=25&t=175 .
RESULT
Calls in work fine. Calls out work fine, though sipgate test numbers (10000) you have to wait about 10 - 20 seconds before you hear anything. And I have no credit so can't say what real call are like, but I still get the sipgate "you have no credit" man so guess it works fine.