Having just been through this (we are going Sept 14-21), here's my experience:
(1) You begin online booking at 7 am EST
(2) You can call to book beginning at 8am EST (but I would recommend calling before that and waiting because there is a "line" to speak to a rep)
(3) The time slots offered online are not the only ones available. You can get different/more options through the phone booking.
(4) Online only gives you 180 days from that day (i.e., it does not allow you to book your entire trip). The lady I spoke with said this is a giltch they are trying to figure out in the system. But, in order to get the dining reserved for your entire trip at 180 days from your first day, you must call.
(5) Some of the restaurants time slots are still unavailable, even over the phone, until it is actually 180 days from the day you plan to dine (my example was The Hollywood Brown Derby - I had to wait until 180 days from the day I planned to eat to get my fantasmic dinner package at a decent time).