Besides the Delivery Status, you may also see Reason of Failures in your Report. Below are some common Reason of Failures and their explanation.
Mobile Equipment Error : This error may occur if the smartphone or normal phone is in an area of poor coverage or loses single while receiving an SMS message. The error may also display if the message sending sequence from operator did not complete before the SMS message was sent to the handset.
Network Error : The possibilities of non delivery report can be that the the sms is not delivered yet due to any network issue however default limit for retrying message is 3 times, if the mobile enters into coverage area within retry period than operator will retry the message and the same will get EXPIRED if there is no active alert signal. Also note that, for OTP routes we have set primary route expiry for less as OTPs can't be in pending state for long.
Absent Subscriber : Mobile is switched off or out of coverage area, retry for this error code will be done based on the active alert signal from the operator and the same will get EXPIRED/FAILED if there is no active alert signal.
Content Blocked : Message failed due to specific keyword(s) in the message content.
Blacklisted : Number is blacklisted at the operator end.
Note that most retry is done based on active alert signal, if Operator does not get any active signal, then the message may fail with no retry.