Yes, it is a TMO phone. New from TMO on Monday with the 18.104.22.1680 software on it. UMA worked with that OS version.
It now has 22.214.171.1244 software. I'm tempted to try the 126.96.36.1991 beta.
If I could find a copy of the TMO official 188.8.131.520 software, I would put it back on the device (no, as of a few minutes ago, it was not on the Unsupported Browser