UMA issues
I too have UMA issues with my Airport extreme at work. I have tried changing the settings, but with little luck.
I have a Linksys router at home which has fewer problems. About once a day, my airpot loses its connection. Was really sure why, but it is losing every voice call.
It seems to work for a while after I change the settings and reboot, I have also changed it so that it does not timeout the WPA key. I also changed the multicast rate a few times, with the only sure result was that a higher one seemed to drop more calls. I am using the lowest setting now and it is working for now, although, I just rebooted. I am not even sure what the multicast rate is and Apple's instructions are not very good about those details...