What are the reasons of Delay of OTP SMS Delivery?

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The most possible reason of delay delivery of SMS is due to SMS is in retry stage at SMSC (Short message service centre) end. Below are the possible reasons why the messages put on retry by the SMSC

SMSC terminates the messages and wait for delivery acknowledgement. In case proper delivery pack not received and error acknowledgement received to the SMSC from far end network; Such SMSs are put on retry. These errors for which retry happens could be SMS/ CALL_BARRED, SUBSCRIBER_BUSY, NoRespFromPeer, Unidentified Subscriber, TimeoutError from far network and Memory Capacity Exceeded etc.