Yes, it is a TMO phone. New from TMO on Monday with the 220.127.116.110 software on it. UMA worked with that OS version.
It now has 18.104.22.1684 software. I'm tempted to try the 22.214.171.1241 beta.
If I could find a copy of the TMO official 126.96.36.1990 software, I would put it back on the device (no, as of a few minutes ago, it was not on the Unsupported Browser