8820 is UMA capable...maybe TMO is not offering that service on the 8820

the software has UMA settings hardcoded which points the device to the correct UMA network when you enable UMA and try to connect, you will also need the correct certificate that your device will need to use when getting authenticated on the UMA network. You can see the settings when you look at the Wi-Fi diagnostics details, it will show you the correct URL of the network UNC and Security Gateway. you need those two key settings in the device + the certificate (certificate can eaisly be obtained from another TMO blackberry, you can just ask your friend to browse through the certificates on a TMO BB and select it and email to you right through the device, but settings cannot be inputted manually)

I have never tested the software theory explained above but I am hoping that will work becuase it makes sense and the device should connect to the correct UMA network.