A few of my friends had the same problem re-filling their AT&T Calling Cards until I showed them how to fix it.
1) Use up all your AT&T Calling Card minutes until your card is empty and worth 0.
2) Use scissors and cut up the card into tiny tiny little pieces and throw away.
3) Sign up for
BigZoo.com which has better domestic (2.9 cents per minute) and international rates than most calling cards, plus you can recharge your PIN online and you'll love their detailed billing.
2.9 cents per minute!
Here's an informative Off Topic thread on
long distance providers.