You can add T-Mobile ONE Plus or T-Mobile ONE Plus International (to only T-Mobile One No Credit Check plans), No Credit Check plans already includes 200MB of domestic roaming. The ONE No Credit Check plan, and 10GB No Credit Check plan do not include Simple Global or any international roaming.
Customers with No Credit Check plans can call countries that do not result in a charge, like Canada and Mexico (with exception of 10GB No credit check plan – available for $10/month), but these plans cannot incur additional charges on their bill, so calling to countries that incur charges is unavailable. You can also add Stateside International Talk (SIT) for $15 per month to your plan and get discounted calling rates to landlines in 70+ countries, and unlimited calling and texting to mobile numbers in 30+ countries. And SIT is now available to you in ONE Plus International for $25/month.