There can be instances where you send messages but fail to deliver them. One most possible cause is where the "From"/"To" pair violates delivery rules. Sometimes, when a user is opted out of the message received from a specific phone number belonging to a message service, they get opted out from receiving any messages from the message service. As a result, this error could occur when such users are involved in the communication.
If the error occurs because of the cause mentioned above, which is most likely the case, the solution is to text "START" or "YES" to your Twilio number to opt-in again.
If it is not the case, and the API Explorer comes up with a 201 response, but no message arrives, verify whether the "To" phone number is active, valid, has good reception, and is not roaming internationally. It is also essential to check whether the number is not on a "do not contact" list.
After checking those factors, try resending the message and if it does not succeed, contact the support team with at least three message SIDs of such attempts.
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