Dude, my 9780 WAS only usable. It's 2013. I have a lumia 920 now. My point is that TMo's service isn't Skype and it's not as simple and straightforward as you're making it sound. Getting a foreign SIM is always the best strategy.
It is that simple...
I travel for my job, and WiFi calling has saved me thousands in phone bills. I've used WiFi calling (UMA) with a BB Pearl 8120, BB Curve 8520, Samsung T339, and Nokia 6136 and (IMS) with Amaze 4g and now the 521.
You enable WiFi calling, and when you're connected to a WiFi hotspot, your phone works just as if it were back in the US. Example, last time I was in India, I made over 300min worth of calls, and at the going international roaming rate, that would have been over $900 (just over $3/min). With WiFi calling enabled, no extra charge.
WiFi calling is completely different from Skype or similar Voip services and requires carrier support to work.