just got a text from AT&T stating the following:

With the latest software update for your phone you can use Wi-Fi Calling when traveling outside the US. This means you can call back to the US or receive incoming calls, with no airtime charges. When you call international numbers, your rates will be the same as those in your smartphone plan or your international package.

apparently this currently only works with the iPhone 6 or newer, running iOS 9.3 which was just released yesterday, 03/21...and without an international roaming plan (ie. AT&T Passport), AT&T charges $2.50/minute for making/receiving "local" calls while in the caribbean

so this new feature is nice for calls to/from the US while travelling, but offers no real benefit for making local calls while overseas