The PhoneNumberValidator class validates that a string is a valid phone number. A valid phone number contains at least 10 digits, plus additional formatting characters. The validator does not check if the phone number is an actual active phone number.

class PhoneNumberValidator extends WidgetValidator {
 
 /**
  * The set of allowable formatting characters.
  * By default equals "()- .+"
  */
 String allowedFormatChars;
 
 /**
  * Minimum number of digits for a valid phone number. A value of null means 
  * this property is ignored.
  * By default equals 10.
  */
 int minDigits;
 
 /**
  * Error message when the value contains invalid characters.
  * By default equals "Your telephone number contains invalid characters."
  */
 String invalidCharError;
 
 /**
  * Error message when the value has fewer than 10 digits.
  * By default equals "Your telephone number must contain at least {minDigits} digits."
  */
 String wrongLengthError;
 
 PhoneNumberValidator(Widget widget, {
   this.minDigits:10,
   this.allowedFormatChars:"()- .+",
   this.invalidCharError:"Your telephone number contains invalid characters.",
   this.wrongLengthError:"Your telephone number must contain at least {minDigits} digits."
   }) : super(widget);
 
 /**
  *  Executes the validation logic of this validator to validate a phone number. 
  *  For an invalid result returns an List of [ValidationResult] 
  *  objects, with one ValidationResult object for each field examined by the 
  *  validator that failed validation.
  *
  *  You must override this method in a subclass of a validator class.
  */
 List<ValidationResult> doValidation(dynamic value) {
   List<ValidationResult> results = super.doValidation(value);

   // Return if there are errors or if the required property is set to false 
   // and length is 0.
   String val = value != null ? value.toString() : "";
   
   if (results.length > 0 || ((val.length == 0) && !required))
     return results;
   else
     return _validatePhoneNumber(value);
 }

 /**
  * Validate [value] and  return List of [ValidationResult] objects, with one
  * ValidationResult object for each field examined by the validator.
  */
 List<ValidationResult> _validatePhoneNumber(dynamic value) {
   List<ValidationResult> results = new List<ValidationResult>();
   
   String valid =  Validator.DECIMAL_DIGITS + allowedFormatChars;
   int len = value.length;
   int digitLen = 0;
   int n;
   int i;
   
   n = allowedFormatChars.length;
   for (i = 0; i < n; i++) {
     if (Validator.DECIMAL_DIGITS.indexOf(allowedFormatChars[i]) != -1) {
       throw new Exception("Invalid format chars");
     }
   }

   for (i = 0; i < len; i++)
   {
     String temp = value.substring(i, i + 1);
     if (valid.indexOf(temp) == -1)
     {
       results.add(new ValidationResult(true, invalidCharError));
       return results;
     }
     if (valid.indexOf(temp) <= 9)
       digitLen++;
   }

   if (minDigits != null && digitLen < minDigits)
   {
     results.add(new ValidationResult(true, substitute(wrongLengthError, '{minDigits}', minDigits)));
     return results;
   }

   return results;
 }
}