Page is a not externally linkable
phranque - 6:47 am on Sep 12, 2012 (gmt 0)
to be a valid number it's got to have 10 digits or a 1 followed by 10 digits.
remove all non-numeric characters, remove the leading 1 if any, if it's not 10 digits, reject the input.
you might go further and validate the area code.