code: def _serialize_call(self, format_, call):
          
          if isinstance(call, record.UnparsedCall):
              return call.unparsed_data
          else:
              result = [
                  format_value(self.header.get_format_field_info(key), call.data.get(key), "FORMAT")
                  for key in format_
              ]
              return ":".join(result)
docs: Return serialized version of the Call using the record's FORMAT