I bought mine from Ascent and I am sure there are other people using it with VOIP providers other than WxC. I might try the suggesting from GKJones of setting up a free 2talk account, letting the auto provision go through and then change the username/proxy etc.
Can you tell me what is the full version of your firmware as this may shed some light on the issue.
As for 2Talk, I set up an account but found that auto provisioning with them only covered the SPA2102 and PAP2T, and neither seemed to actually provision my router. I went into the help system and manually configured it based upon the flavour of the Linksys models, keeping my iTalk specific settings, but that still gave the save symptoms.