Yes, it is a TMO phone. New from TMO on Monday with the 188.8.131.520 software on it. UMA worked with that OS version.
It now has 184.108.40.2064 software. I'm tempted to try the 220.127.116.111 beta.
If I could find a copy of the TMO official 18.104.22.1680 software, I would put it back on the device (no, as of a few minutes ago, it was not on the Unsupported Browser