Return serialized version of the Call using the record's FORMAT

def _serialize_call(self, format_, call):
        """Return serialized version of the Call using the record's FORMAT'"""
        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)