AWS SNS Text Messaging(SMS) did not deliver message to intended user even though the status is SUCCESS

We use AWS SNS Text Messaging (SMS) service to send messages to our clients in our app. We have moved the service out of Sandbox and increase the quota to cater for production usage. We also enabled Delivery Status Logs to monitor the message delivery.

In some cases, the messages were sent successfully. However, in some cases, the messages were not delivered to the intended phone number. When we checked the status logs, the indicated status was SUCCESS for those messages and AWS charged them.

Does anyone encounter the same issue? How do you ensure the reliability and consistency of SMS sending service?

Thanks