Just spent hours with this, and looking through posts and finally got the thing working! Thought I'd post this to let people know the magic trick...
I had no luck with STUN as per some previous posts. What did it for me in the end was:
Get the latest firmware from Grandstream.
Configure a static IP for the phone (You don't have to, but my LAN is complicated!)
Forward ports in router to phone.
For the login details, i used my 6 digit SIP number in both fields. I had no luck with 'plustalk' for some reason.
Now, here's the crunch part. If your ports are forwarded correctly, leave the NAT traversal setting to 'No' BUT then further down the config screen there's a space for 'use NAT IP'. Put your *public* plusnet IP in there. That way the phone sends out the correct messages to the SIP server.
I'd spent hours with NAT, sipgate STUN server, etc.... but finally it was the no-NAT-but-tell-me-your-public-IP approach that worked.
Hope this helps someone else to keep some hair!