T-Mobile's "Canada and Mexico are considered domesic!" isn't really true.

Most American cellular companies now offer data and calling plans where Canada and Mexico enjoy a special roaming status not bestowed on other international destinations. If you read the ads, you're generally led to believe that usage in our two nearest neighbors is considered "domestic" – even at full LTE speeds. For most one- or two-week vacations, this is effectively true.

I'd heard rumors online that people on longer visits had their service throttled or completely cut off by T-Mobile if the majority of their cellular usage in a single billing period is Canada/Mexico roaming. Well, I just got that dreaded text today, March 14th. For the record:
My billing cycle resets on the 22nd of each month.
I got to Mexico Jan 26th, and have been back in the states since March 4th.

Now, at this point, I don't care if they throttle my Can/Mex roaming for the rest of this billing cycle because I'm back home, but I am curious what would have happened if I'd been there longer. There's lots of conflicting information online about what triggers this warning, what the actual shutdown conditions are, and whether or not Canada/Mexico roaming is in the same "bucket" as all of their other international roaming options.

I reached out to T-Mobile directly and I don't think their Customer Service representative even really grasped the rules. If you look at the Terms of Service, you can see it leaves them a wide berth on how to interpret "50%":

Unless explicitly permitted by your Rate Plan or Data Plan, you are not permitted to use your Device or the Services in a way that we determine results in more than 50% of your voice and/or data usage being Off-Net (i.e., connected to another provider’s network) for any 2 billing cycles within any 12-month period

Their rep told me it has nothing to do with minutes or megabytes – the "50%" is based on the number of days that your phone is using a foreign network. She advised me that leaving your phone off, in airplane mode, or with all roaming disabled for 24 hours would prevent you from "using up" one of your roaming days.

This seems at odds with everything I'd read online and then it dawned on me: there's probably some kind of automated algorithm that flags your account for excessive international usage, but a human being then reviews your account by hand. This would explain all of the conflicting stories about what the rules are. Furthermore, she told me the restrictions are per-line, not per account. Lastly, I'm not sure if this means anything but she also added, "Our Canada/Mexico roaming options might very well change before you go back to Mexico next year." 🤷‍♂️

The point of my worrying about all of this is that we're going to Japan and Hong Kong for a month in May, and if I have to spend the whole time fretting about roaming, it really defeats T-Mobile's #1 selling point.

After ruminating about it for the past 24 hours, I think what we'll do is roam while we're in Taiwan and Hong Kong (total of 7 days) and then rent a Mi-Fi again while we're in Japan.

I'll be honest, the "free" T-Mobile internet is great for being able to do messaging throughout the day, but even if you pay the extra $15 a month for their "double speed" option, it's only 256 kb/s – slightly faster than 2G. It's annoying to use for anything much beyond iMessage (or Line or WhatsApp). Given that Japan maddeningly doesn't permit you go download and cache Google Maps of their country, slow internet service becomes extra irritating.

(Sidenote: in Japan it's forbidden to issue SIM cards with callable phone numbers to foreigners. So if you decide that swapping your SIM is a route you want to go, make sure you have a Skype or other voice calling option figured out before you go)