code: def _maybe_append_formatted_extension(numobj, metadata, num_format, number):
      
      if numobj.extension:
          if num_format == PhoneNumberFormat.RFC3966:
              return number + _RFC3966_EXTN_PREFIX + numobj.extension
          else:
              if metadata.preferred_extn_prefix is not None:
                  return number + metadata.preferred_extn_prefix + numobj.extension
              else:
                  return number + _DEFAULT_EXTN_PREFIX + numobj.extension
      return number
docs: Appends the formatted extension of a phone number to formatted number,
      if the phone number had an extension specified.