For T-Mobile One customers, the One Plus worldwide option can be added for an extra $25 per month, and that'll get you unlimited high-speed data when venturing across the border. This change not only affects T-Mobile ONE customers, but those locked into an older Simple Choice plan as well.
With T-Mobile ONE and ONE Plus the limit is 50 gigabytes. What happens if you reach that 5GB limit?
T-Mobile has stopped its unlimited data packages in Canada and Mexico, and slapped a cap on its LTE data. After burning through 5GB, your speeds will drop to just 128kbps (or 256kbps if you have a T-Mobile One Plus plan). T-Mobile says it will throttle the customer to 128Kbps with the exception of T-Mobile ONE Plus customers, who will receive 256Kbps speeds.
Go from NY to Canada, or from California to Mexico; it's just like going from Idaho to Texas, from Georgia to Kentucky.
In the summer of 2015, T-Mobile announced its "Mobile without Borders" Un-carrier movement - a program that introduced unlimited calling, texting, and LTE data to Mexico and Canada for subscribers in the U.S. Mobile without Borders isn't going anywhere, but T-Mobile is making a pretty big change to it that does cut back on its value considerably. For example, if you use 2GB of LTE data in Mexico or Canada during March, you'll have 3GB remaining.
T-Mobile customers can track their LTE usage in Canada and Mexico by dialing the short code #WEB# on their phone. After using the 50GB of LTE data you'll likely be slowed down to a crawl, especially during the more congested hours. For customers who already have a data limit, not a lot is changing, but the Data Stash option that lets you roll over unused data can't be used in either Mexico or Canada. In case you do not want to pay the extra fees to T-Mobile, you can purchase a new SIM card if you are in these countries for a longer duration of time, however, you would have to manage with a different number.