I'm having the exact same issues. I posted here
where someone redirected me to this thread.
I've been going around and around with T-mobile about issues with UMA, but Friday I realized that the bluetooth being connected was the source of the problems. I verified this on my network (WRT54G-TM), a T-Mobile store hot spot, and at work (WRT54G). All places it worked fine with no headset connected and cannot make or receive calls with the headset connected.
I tried turning encryption off as suggested above but it did not help.
I only have one week left on my 30 day trial period of service with T-Mobile so I need to decide if this is going to be a deal breaker or not. My biggest need for UMA is at work where the T-Mobile GSM signal is too weak to penetrate the building. AT&T's signal comes in fine, however since AT&T stopped paying T-Mobile to use their tower, service at my apartment went so far downhill I can't use them any longer. T-Mobile's signal is much stronger, however in the back of my bedroom I often drop service.