If I recall, you have to enter a number ver in the format XXX XXX XXXX, and the first number cannot be a 0. To get around it, I just entered my mobile number, transposing the 0 at the start to the end of the number. They'd never be able to reach me on the number, but with My Disney Experience and all the details they have on file with your reservation, I don't think it would ever be a problem. At least it allows you to make the all important ADR.