I have the One Plus promotional plan. Up until my last international trip last month, I would always get the following welcome message:

"Welcome to United Kingdom! ! T-Mobile ONE Plus gives you unlimited data at 2X the speeds of our T-Mobile One plan, up to 256kbps, calls at 20 cents/min, and free texts. Learn about Roaming here http://t-mo.co/intlroam"

Always made me a bit nervous that it was a promotional message for the One Plus and somehow they had not turned on the faster speed for my line (even though my speeds seemed about right.)

On my last trip in September, the message had changed and is now clearer:

Welcome to United Kingdom! Your T-Mobile ONE Plus feature gives you data at 2X the speeds of our T-Mobile ONE plan, up to 256kbps, unlimited texts, and calls at 25 cents/min. Visit t-mo.co/intl-plus1 to learn more and view additional data options. Enjoy your stay!

I'm always amazed at how much actually gets done at these slow speeds - e.g., Whatsapp voice calls are flawless.