Earlier quoted context omitted.
To be blunt, inputs you don't validate are inputs that will be entered wrong a significant portion of the time. Other concerns may override this for many fields, but unusable phone numbers are generally considered a problem.
If you force people to enter a phone number in a format they don't have, you will just end up with a bunch of 555-555-5555 numbers. Either way, you are going to get incorrect information. It's much better to realise that your information will not always be 100% accurate no matter what you do.
I'm very much for allowing people to enter any format, but seeing how badly people mis-enter NANPA phone numbers, it is simply wrong to not acknowledge that you will see far more mistakes from an open input box.