When I was first configuring Sipdroid, I kept getting a notification from Sipdroid "Registration failed (404 not found)". I thought this meant that Sipdroid couldn't find the pbxes.org server and I tried configuring it to talk directly to my SIP service (which didn't work, either).
It turned out that the 404 error was extremely misleading. What it was really indicating was that my Trunk configuration in pbxes.org was non-functional. I had not correctly configured the Trunk to correctly access my SIP account. Once I read the instructions from my SIP provider and got the trunk to authenticate the 404 error disappeared.
The error should really be something more like "401 trunk not authorized".