Roaming and international calls on mobiles are horrendously expensive.
I understand the EU (or some bit of it) has the matter under review.
Some providers (I don't know about T-Mob) offer a "reduced price" deal, either free if you register, or bigger discount if you pay a fixed amount per month.
Still expensive, but no longer horrendous.
Regular travellers seem to have a wallet full of PAYG SIMs for the locations they visit. It makes local calls a lot cheaper.