While sending MT-SMS, I found error code 6 which means, the subscriber didn't respond to the MSC paging and MSC reverted with error code "absentSubscriberSM".
This might be due to low signal strength, or device is in an idle state or powered off. If a device tries to re-register again with the network and location update is successful and then we can try to send an SMS.
Resolution: reboot or power cycle this device.