OK since I've already wasted my Friday night, I might as well lay out
what I've found.
I've successfully changed the firmware to two other versions, a .10LSc
and a .13LSb.
It appears thought that the provider config is stored somewhere
outside of the main firmware, because despite flashing to different
versions, I am still prompted to enter a password for the admin
pages, and the device still makes requests to a vonage tftp server.
I tried a factory reset after loading each firmware, and it didn't
help.
I noticed that the device says it has a certificate installed. I'm
assuming this is what's used to authenticate/decrypt the .xml config
file the device is trying to load. If that's the case, then the
configs are likely signed with a key unique to vonage, and that
pretty much ends that direction. I think that will likely prevent
the loading of some generic, yet properly compiled config file, since
it won't be signed by vonage's key.
I read somewhere that older versions of the firmware had a particular
vulnerability that allowed config access - does anyone recall what
that was about?