U.S. wireless roaming is free on all nationwide T-Mobile plans. While no additional fees are incurred during such roaming, these minutes are treated like regular T-Mobile minutes.
For example, a U.S. roaming minute at 3 p.m. on Tuesday counts as a regular anytime minute whereas on Saturday it would count as a free night-and-weekend minute.
International roaming minutes, on the other hand, are billed as separate minutes of use. These can be pricey with T-Mobile and can go up to $4.99 per minute. Such fees by country can be found here. Read more roaming policies by carrier here.