I've just returned from the T-Mobile Store in Midland Park, NJ. The young woman there, who seemed well-informed, and not particularly like a stereotyped predatory salesperson, said that our phones would connect to the networks in foreign countries. But she said that unless we purchase, temporarily, T-Mobile ONE Plus, we will pay something like $3 per minute for calls. (I don't care what my data speed is while traveling.)
I have no way of verifying precisely what she said. I will agree that the quote below (from online expansion of T-Mobile ONE Plus details), does not mention rates for data, texts, or voice minutes. I am inclined to believe the previous posts by experienced users!
"One Plus gives you HD streaming, 20GB of high speed mobile hotspot data, 2X the data speeds abroad compared to T-Mobile ONE in 210+ countries & destinations, unlimited in-flight data on Gogo-enabled flights to, from, or within the U.S., Visual voicemail, and Name ID. On all T-Mobile plans, during congestion, the small fraction of customers using >50GB/mo. may notice reduced speeds until next bill cycle due to data prioritization."
T-Mobile ONE Plus costs $15 a month, monthly until canceled online or in person. She said that cancelling T-Mobile ONE Plus, after a month or two, would not harm my right to my previous (Unlimited 55) plan.