I'm not really sure exactly what it means but I know that it has something to do with using the data plan/wi-fi to make calls over the internet. Will the Essex be able to do that?
Also, does the existing Tour have this capability?
Not sure of the Essex, but as was mentioned the current Tour doesn't have WiFi hence can't support (from technical reasons) any UMA capability. You sound like UMA is a bit of a mystery to you, maybe read the Wikipedia article for a clear mind?
UMA seems to be GSM device supported only right now, no CDMA. While the Tour and Tour2 are world phones capable of using GSM I don't think the US carriers/device would be able to pull off the hand off from CDMA - GSM - UMA.
UMA is great, I have it enabled on my 8520 and I use way fewer minutes because of it.
