docs: def callback(request):
      """Callback URL for Nexmo."""
      message_id = request.GET.get('messageId')
      status_id = request.GET.get('status')
      status_msg = NEXMO_STATUSES.get(status_id, UNKNOWN_STATUS)
      error_id = int(request.GET.get('err-code'))
      error_msg = NEXMO_MESSAGES.get(error_id, UNKNOWN_MESSAGE)
  
      logger.info(u'Nexmo callback: Sms = %s, Status = %s, message = %s' % (
          message_id,
          status_msg,
          error_msg
      ))
  
      # Nexmo expects a 200 response code
      return HttpResponse('')