Actually, i'm speaking for anyone using a US-based phone as a phone. And what I said still applies. Using wifi and a voip app (I don't know UMA, but I assume that that's what it is)
Nope, UMA isn't an app. It's more deeply integrated into the phone software. The phone can attach to either the cell tower or Wi-Fi, and it makes calls over the "best" network. Whether it's connected over Wi-Fi or not, there is no change to how you make calls. Although it is VoIP, it is done from your phone and you dial your phone just like you would in the US. I dialed both the EXP desk and a conference call earlier today by picking up my T-Mobile USA phone and dialing a number. Both went over Wi-Fi because my hotel has Wi-Fi.
All that said, your UMA phone has to come from T-Mobile because they're the only US carrier that supports it, and you have to get within range of a Wi-Fi signal you can use.