de_... is correct - just wanted to make sure your fears were put to rest, but one bit of caution:
*Texting* on the other hand is *NOT* necessarily free between users of the same network, but voice calls are. (In the US - if one of you has T-Mobile in the UK/Europe and the other is in the US, I won't place any bets - I'm almost certain it's only between US customers..) Also, I don't know how things work with T-Mobile in Europe, even if you both have T-Mo - both De.. and I are only talking T-Mo USA here...