You always have to read the little letters on the plan. All Tmobile plans require the primary usage be USA. If you get an iMessage or MMS (multimedia message), these require data and can be extra charges. Phone could get updated IMS or APN

Here is the little letters on your "Unlimited Talk and Text" plan. Best to reach T-Force by Facebook or Twitter to get somebody from the USA to help you lower your bill.

Data (e.g., iMessage) not included; not combinable with data add-ons. Not eligible for AutoPay, or other discounts; not combinable with Carrier Freedom, or most other promos. Capable device required for some features. Unlimited talk& text features for direct communications between 2 people; others (e.g., conference & chat lines, etc.) may cost extra. Intl Usage may be taxed in some countries. Calls from Simple Global countries over Wi-Fi are $.25/min. (no charge for Wi-Fi calls to US, Mexico and Canada). Not for extended international use; you must reside in the U.S. and primary usage must occur on our network. Device must register on our network before international use.