When on UMA, the UMA will automatically transfer to EDGE when you leave the wifi area. So, the device has to sense the signal weakening *before* you leave the area in order to make the transfer to EDGE. It must have some sort of signal threshold where it says, Ok, you're leaving wifi, I'll transfer your call to EDGE. So, in weak areas you will likely see UMA go back and forth from UMA to EDGE.

That's where the travel router comes in handy. The BB senses a nice strong signal all the time so the UMA connection doesn't drop. However, if signal weakens a little to the travel router, the call may get distorted or drop due to interference, low bandwidth, or low signal.

None of this is scientific, just my observations when using UMA. That's why boosting the power of my wireless router at home (after installing DD-WRT) had such a profound effect on UMA.