FYI,
For anyone hoping to start a T-Mobile plan in order to travel abroad, we just had a major problem and wanted to alert other travelers.
Went to T-Mobile store today to open an account for a family member to use on 4 week Europe trip starting next week. We were told that you have to first use the account in the U.S. at least as long (if not twice as long) as you intend to use the line abroad. Otherwise, T-Mobile will suspect fraud and will disconnect you (or require complex administrative verifications that will be impossible for our family member traveling abroad). So it won't work for us. (Had wanted to do T-Mobile because crossing through multiple countries and seemed too complicated to switch SIMs repeatedly.)
Tried talked to manager who seemed pretty clueless but said she would pass the word on to higher-ups. I explained that I had heard great things about T-Mobile and was going to spread the word that there are not as easy to use internationally as we had been led to believe.
Hope you all can use this advice. I had been doing research on international options for months but never came across this.