From what I have read...the Airport Extreme doesnt play well with UMA right?
However, I bought an Airport Express and use it at work when I want wireless for my laptop and it keeps UMA until I turn it off.
Maybe it's because of the Extreme runs an N network?
What exactly happens when you are using the non-N routers?
I had this problem for a while (back when I was using my old as hell Netgear router), but changing routers fixed it. I hope for your sake you won't need to change routers!
Have you also tried to contact customer service? the T-mo customer service is pretty good about how to solve things..if it's minor and something they have a record for. In your case, I haven't seen much about it in these forums so it might not be such a well documented case.
Hope you get it to work out!
edit: Blackberry Feature Enabler - UMA
Try calling Cust.Serv.and asking for the UMA feature enabler?