Have you tried to download the phone configuration file manually to see if the file is actually present?
from win: tftp -i x.x.x.x get SEP.cnf
If you can download the file, I would suggest you examine by hand, it's XML and fairly human readable, ensure that nothing looks obviously corrupt.
Other ideas:
* did you load the onto the system via BAT? I have seen imported phones with essentially corrupted resulting TFTP files in certain cases.
* have you enabled security within your cluster? doesn't sound like it (you should get an "auth" error instead)
* is auto-registration enabled on the cluster?
If all else fails, you might try deleting and readding the phone manually via the web admin interface.
