code: protected String getReceiver(final SmsSendRequestDto smsSendRequestDto) {
      if (StringUtils.isNotBlank(smsSendRequestDto.getReceiver())) {
        return smsSendRequestDto.getReceiver();
      }
      Validate.notEmpty(defaultReceiver, "defaultReceiver must not be null or blank");
      return defaultReceiver;
    }
docs: Returns the receiver of the request. If no receiver is specified the default receiver will be
  returned.
  
  @param smsSendRequestDto the sms request
  @return the receiver
  @throws IllegalArgumentException if no receiver is specified at all