I do most of my ADR's thru MS, but once I was told by them that there was nothing available at one of the places I was trying to get. Not wanting to give up, I got off the phone with MS and called WDW-DINE. They were able to make the ADR for me.
They both use the same reservation system. By calling MS, you can use a toll free number. The main dining line opens two hours before MS and is open on holidays.