Is there any way to auto resend failed SMS due to network problem like telco's traffic congestion?

In the scripts directory of a package there is an example script smsresend. You can use cron or regular_run to run this kind of script periodically. This script uses a counter for retries and by default it will retry 5 times.

A definition failed = /var/spool/sms/failed in the smsd.conf should be used to get this working.

This script could be modified to check the Fail_reason header, and in case of a permanent failure, for example nonexistent destination number, retrying could be skipped. Also it could check when the latest trying was done, and retry after 30 minutes, for example.