code: public static boolean isInternationalPhoneNumber(String s) {
          if (isEmpty(s)) return defaultEmptyOK;
  
          String normalizedPhone = stripCharsInBag(s, phoneNumberDelimiters);
  
          return isPositiveInteger(normalizedPhone);
      }
docs: isInternationalPhoneNumber returns true if string s is a valid
  international phone number.  Must be digits only; any length OK.
  May be prefixed by + character.