What is a Retry?
In the context of SMS, retry refers to the process of the SMSC trying again to deliver an SMS message that failed to be delivered – during a period the message is valid, usually 48 hours.
If the delivery of a message fails, the SMSC (Short Message Service Center) starts a retry procedure that tries to deliver the message periodically. Voice message delivery can also be retried during the message validity period (up to 48 hours).
How does retry work?
When we send a message, we expect it to be delivered within seconds. When this doesn’t happen, the SMSC takes several measures to ensure a reliable service, including retry. Failed SMS delivery can occur for various reasons, either temporary or permanent.
Two actions can be performed when a temporary problem occurs: re-sending when the handset is available or a time-based retry.
Temporary errors are retried more frequently, such as communication failures (i.e., every minute for five minutes, then every 10 minutes for an hour, then every hour for 24 hours).
Long-lasting ones, such as absent subscribers, can be retried every hour for seven days.
Suppose the SMS delivery failure is permanent (most likely because of an invalid MSISDN), no additional retries are attempted, and the delivery notification is sent to the sender to acknowledge that the delivery of the message has failed.