The first file the Bios will look for is dependent upon the bios itself. Most often the bioses are set to look for evoxdash.xbe first, then a few other dashboard replacements, then finally xboxdash.xbe.
This could be different IF the bios you have has been altered to boot something else first.
Don't know which other bioses phoenix is compatible with (should be in the .nfo for it) but if you have the right RC4 key and 1MB bios, then you can feasibly change over to another bios instead of the default one it comes with.