Start calling at 6:55 or so. I got through and made a ressie and it wasn't even 7 a.m. yet. They sell out by 7:03 or something like that.
I also dialed on both my cell phone and my land line. You may want to recruit others to call as well.
Another tip is to wait and speak to a CM even if you are on hold. You may be first in line. Also, duplicate reservations get cancelled and sometimes something opens up. I called ten days later to see if I could make a later PS and there wasn't anything available that day but there was the following Sunday. You can call back even after the 90 day window. The toll free number is 1-800-828-0228, but I wouldn't use this on your initial try - only after the 90 day window has passed.
Good luck!
Diana