Disneychrista is right, they sell out almost INSTANTLY!
What I did was call a minute before and then kept redialing until I got through.
Then I said "Fantasmic Dessert Buffet" and the date, time and number of people and credit card number.
I found that if you speak fast with a sense of urgency they type fast. If you give them a pleasant "good morning" they type slow and you wont get in.
So, my advice is to be a bit rude and apoligize after you get in!