How can I unlock a Blackberry Pearl 8110 and a BlackJack II

my cousin just bought two brand new phones. one is the blackberry pearl 8110 and the other is a blackjack II. both of these are AT&T phones; the problem is that my cousin has service with T-Mobile. How can he unlock these two phones to be able to use them on T-Mobile? Does he need to talk with AT&T or with T-Mobile? Is this even possible?