The Fantasmic packages can be booked up to 30 days in advance, just make sure you let them know which one you are intereseted in as there are two options and they cost the same : There is the Balcony viewing (with assigned seating...based on when you book) up on the Gallery balcony and there is the Premium viewing which is first come first serve (of those who booked) to the area between the French Market and the Rivers of America.
The phone number to call is 714-781-4400 (between 8am and 5pm daily)
**this is also the same if you are interested in the Golden Vine Winery Seating for the Electric Light Parade.
If you are just looking to book regular dining reservation for resturants within the parks/hotels you would call 714-781-3463 between 8am and 9pm daily and those priority seatings can be made up to 60 days in advance.