When the DTA310 connects to your local Asterisk, do you use numeric extensions only to register to it?
If yes, I believe the DTA310 either cannot support alphanumeric SIP URIs or it cannot support the dash (-) between ericp56-xxx and the extension number.
The dial-tone you hear, is most probably created locally in the ATA, regardless of its' registration state.
In order to figure this out, you need to run a trace for all packets off the DTA310.
Yeah, I gave up on it and bought a Grandstream HT. I config'd a local asterisk and traced packets, to figure out how to set up the 8x8 adapter, so it would register, and then I switched to pbxes.org to no avail. The HT is well worth the $30. I was up and running in minutes.