as with any phone that uses a simcard, you'll either need to call t-mobile customer support and get international roaming added to your plan (this can be expensive), or a cheaper solution when traveling to another country is to buy a cheap phone that's unlocked (if your phone isn't already unlocked) and then buy a prepaid sim card in the country you are visting.
if you only plan on making local calls in the country you are visiting than buying a sim card in the visiting country is much cheaper.
if you plan on making calls back to America then it's probably better to get international roaming added to your plan for a month. just be aware that calling back home to America can cost you about $3 per minute. If you buy a euro sim card and keep calling to America your sim card will run out of minutes very fast and it will be a head ache constantly having to go buy new sim cards.