Yes, it is a TMO phone. New from TMO on Monday with the 126.96.36.1990 software on it. UMA worked with that OS version.
It now has 188.8.131.524 software. I'm tempted to try the 184.108.40.2061 beta.
If I could find a copy of the TMO official 220.127.116.110 software, I would put it back on the device (no, as of a few minutes ago, it was not on the Unsupported Browser