I've used UMA successfully in Asia (except not tried Korea yet, and that's CDMA market - in theory it should still work as not connected on gsm network). The first time I tried (HCMC), even disconnected the network radio so that I was only connected WiFi. No problems, no international roaming or other fees, it's free.

You need to make sure the UMA icon pops up. However, recently at home I only get the uma (lower caps) but still the minutes are free.

Prior thread with more details: http://www.flyertalk.com/forum/trave...countries.html