code: public static boolean isUSPhoneMainNumber(String s) {
          if (isEmpty(s)) return defaultEmptyOK;
          String normalizedPhone = stripCharsInBag(s, phoneNumberDelimiters);
  
          return (isInteger(normalizedPhone) && normalizedPhone.length() == digitsInUSPhoneMainNumber);
      }
docs: isUSPhoneMainNumber returns true if string s is a valid U.S. Phone Main Number.  Must be 7 digits.