Assuming the phone and router are working OK, has anyone factored Internet connection quality into the equation? I have had the same, apparently random, loss of UMA although the connection to my router is strong (TMo D-link).
Spoke with a TMo/RIM CSR a couple of weeks ago and he directed me to the following site to test speed, jitter, and signal quality, which he said affect the UMA connection: MySpeed VoIP quality test, test VoIP connection quality
(It looks like you can perform about 10 tests before you're blocked for a certain period of time.)
As it turns out, I get poor signal quality to the Chicago location (which is closest to me) but strong signal quality to the other sites.
Just thinking out loud here, but one hypothesis could be that bad UMA connections result from connecting to an UMA service server along a route with poor signal quality. A battery pull could cause the BB to connect through a different server and restore the connection. Likewise, there may be other triggers (web browsing, finishing a call, etc.) that will cause a BB with a working UMA connection to connect to a different server and/or through a route with poor signal quality.