I spoke with Tmo customer service last night. I had the same problem. UMA worked right out of the box (super cool), but failed after a short time ( a day or less), and it took me a while to call them.
There's a bug. (surprise?)
You need to go to "manage connections", then uncheck wifi - wait until the wifi icon disappears from the homescreen (which shows in the background), and then recheck the wifi checkbox.
That should re-force a connection via UMA. The first time I did this, I had to pull the battery for a minute, but since then, the uncheck/recheck has worked to connect me to UMA.
Unfortunately, my UMA connection never really lasts that long (for voice, lasts just fine for data). So, I find that before I make long calls, I try to get on UMA (I have hotspot at home, so my wifi minutes are free). And, fortunately, I live near a good signal, so I'm not dependent on UMA for a connection.
Good luck,
David