Why does my sim card only work in a Blackberry?

I have a Blackberry 8700c. I got it from my company I work for. They pay the bill. I've had the phone since before AT&T bought out Cingular. So the sim card is still a Cingular sim card. Does anybody know why when I put the sim card in another AT&T or Cingular phone, I can only make calls? I can't use the internet unless the sim card is in a Blackberry. I've put my sim card in my girlfriend's Blackberry Bold and it worked fine. I've tried it in a Samsung Blackjack and can only make calls. Any ideas?