I have had good UMA coverage since two nights ago. It
appears this issue has been resolved. If you're not getting UMA sometimes turning off all connections and turning them back on will fix the problem. If that doesn't do it, remove the battery and reboot the device. If that still doesn't do it, erase your WLAN profile using Manage Connections >Wifi Options >Highlight the profile and delete it. Then scan for networks and re-add. Beyond that, if it still doesn't work check your router settings and/or call TMobile. Also check
Main Page - BlackBerryFAQ for more wifi/uma settings, hints and tips.