I think your a bit confused. Yes you can make calls using UMA but you still need T-mobile or another company that supports UMA. When you call someone off of your Blackberry using UMA what number would come up if you turned off all of your mobile connections? Kinda got to think about it for a second.
UMA still uses your connection from your mobile provider. This is the reason AT&T users cannot use UMA even with an unlocked phone from T-mobile.
"how does UMA connection work"
From your perspective, you get one phone number that will work over your Internet connection when you are at home, and over cell towers when you are outside. If you travel to another country, you could possibly make calls to your home town and not be charged long distance. However, UMA is not skype. Other than that exact situation, you will normally be charged for long distance calls depending on your plan.
Normally your phone sends its signals to a cell tower, which forwards it to a server on the carrier's network. With UMA, your phone logs into the carrier's network through your internet connection and sends its signals directly to that server. That means you can access all of the same services over UMA, like voice, data, and SMS. Unfortunately, it means that all of those same services go through your carrier's network, perhaps unnecessarily. Some phones support something called "Internet Offload", in which UMA is only used for voice calls, but all the data goes directly over the Internet. Got it? "
Found this from
UMA Questions Answered - Steve Hanov's Programming Blog 
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Using UMA calling overseas 
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