As a person who have traveled to more than 6 different countries in the last 6 months, I can tell you which U.S. carriers are the best for getting data and voice overseas.
Before I begin, I have service on all 4 major U.S. networks including AT&T, Sprint, T-Mobile, and Sprint. I also have an account with Google’s new Project Fi. Why so many networks? That’s what I do for a living, test all the networks and find the best for you guys and I am always “high” on Android.