When I made my ADR's for August, I woke up 15 min beforehand (6:45 am EST) in order to make sure I didn't sound groggy while talking to the CM. haha! At 6:59 am, I started dialing the dining hotline. You'll get a pre-recorded message saying "Sorry, our office is closed...." or something to that effect. I would just hang up and dial right back. Somehow I timed it right and one of my calls connected right at 7:00 am, you should hear a chime-sound which lets you know the lines are open. You'll then have to go through a series of prompts, such as for English press 1, have you been to WDW before, .... since 2005, are you making ADRS for 8 or more people and for Victoria and Alberts. It's even better if you can guess what the prompts are and cut off the recording. I was put through immediately.
As other posters have said, HAVE BACKUPS. I think it's a good idea to have 3 choices of restaurants you might want on a certain day, along with a time frame (mine was 6-7 pm). And if you don't get exactly what you want, keep calling back, especially at 45 days prior to your arrival, since that's when vacations have to be paid-in-full and people might be cancelling their vacations, and subsequenty their ADRs!