ashtonaut: Awesome, I really appreciate any other thoughts or tests you could help with. The fact that the log shows a failed connection attempt makes me wonder if it's something at 2talk's end that's causing the issue. I have an open ticket with them to (hopefully) investigate.
The only other things I can think to try are 1) Create a new 2talk account with a fresh 028 number and see if I can get that working, or 2) if someone can 'loan' me account details for another VOIP provider (not 2talk) then I could try them out - if they work it would rule out a device issue.
A hiccup. We do have an HG659B at work but it's boxed up and I can't swap out the current one which is an HG659. I thought the HG659B was already in use.
However, I can take the HG659B home and have a play to see if I can get it working if needed. Might not have results today, though due to other commitments.