Another option is to purchase an A.T. & T. prepaid phone card at any Target or Wal*Mart store. Not the most economical, but it's good to have one to carry in your wallet, as the card will also work Stateside.
To use the card, you'll dial the international toll-free access number:
0-800-04114114, then dial the U.S. toll-free number listed on the card, then your card number, then the area code and number you're trying to call. It's like 42 digits...
Sportsmen's: If you're a guest there, they offer direct-dial VOIP calls from your room for very little dinero... I think it's like 20 cents/minute.
I always carry the prepaid calling card, bought in the States. Works for me. The ICE cards also come in a version with an electronic chip in them; these can be used for instance with the pay phones at the HDR.

No numbers to enter other than the phone number. Nice! Sadly, there aren't yet a lot of the new card-reader phones...at least few that I've located. YMMV.