Note that you can also use iMessage and FaceTime from your MacBook as well. There's also FaceTime Audio for voice-only calls over the data connection. I use FaceTime and iMessage when communicating with any iPhone-using friends and Skype/Viber/WhatsApp/WeChat/LINE/Google Hangouts for communicating with friends who don't. I also use Google Voice, which is now part of Google Hangouts, to call regular phone numbers around the world. Calls to U.S. and Canada numbers are completely free.

T-Mobile's free roaming data is likely to be too slow for any sort of video, but may be barely tolerable for FaceTime Audio, Skype Audio, or Viber calls. You can look at data roaming plans with other carriers, which give you a certain number of mb of international data roaming per month at a much lower price than if you didn't have a plan. Another option to consider is getting pre-paid SIM cards in different countries you're traveling to. I have a grandfathered unlimited international data plan from AT&T that's a lifesaver, as I average around 1.5 gb of roaming data per month (much of which is now FaceTime). Even with the unlimited international data plan, I have separate SIM cards for a handful of countries.