I had the same problem. I haven't figured it out either, but I did come up with a workaround:
I configured a static IP address on the IP0020, and configured it *not* to register with the SIP server. Then I configured Asterisk like this (the IP address is 10.0.0.249 and the extension number is 12) --
Dial tone, inbound/outbound calls, you name it. Also important: G.711u codec does not work with the Asterisk package that ships with Ubuntu. I don't know if it's because it's a bad package or because it's Asterisk 1.4, but I built a new Asterisk 1.6 from source, and that cleared up the problem.
Happy to discuss further if you want to ... if I'm not around here, you can find me at http://uncensored.citadel.org
in the "I Want Broadband Everything" and "Telecom" rooms.