Assuming you are on a plan that supports/includes intl roaming - just turn on the phone. I don't use Apple products, but I assume they would work almost as seamlessly as Androids in this regards. Some phones want you to turn on "data when roaming" under settings. There might be a 1-4 minute delay before phone connects with a system. Only use wifi calling when calling U.S. numbers (free calling).
Manage your T-Mobile account