code: def format_national_number_with_preferred_carrier_code(numobj, fallback_carrier_code):
      
      # Historically, we set this to an empty string when parsing with raw input
      # if none was found in the input string. However, this doesn't result in a
      # number we can dial. For this reason, we treat the empty string the same
      # as if it isn't set at all.
      if (numobj.preferred_domestic_carrier_code is not None and
          len(numobj.preferred_domestic_carrier_code) > 0):
          carrier_code = numobj.preferred_domestic_carrier_code
      else:
          carrier_code = fallback_carrier_code
      return format_national_number_with_carrier_code(numobj, carrier_code)
docs: Formats a phone number in national format for dialing using the carrier
      as specified in the preferred_domestic_carrier_code field of the
      PhoneNumber object passed in. If that is missing, use the
      fallback_carrier_code passed in instead. If there is no
      preferred_domestic_carrier_code, and the fallback_carrier_code contains an
      empty string, return the number in national format without any carrier
      code.
  
      Use format_national_number_with_carrier_code instead if the carrier code
      passed in should take precedence over the number's
      preferred_domestic_carrier_code when formatting.
  
      Arguments:
      numobj -- The phone number to be formatted
      carrier_code -- The carrier selection code to be used, if none is found in the
                phone number itself.
  
      Returns the formatted phone number in national format for dialing using
      the number's preferred_domestic_carrier_code, or the fallback_carrier_code
      pass in if none is found.