This issue involves the Twilio "SMS STOP filtering" mechanism where a person can use the "STOP" keyword to unsubscribe and stop receiving new messages from a particular Twilio phone number. This error occurs if the "To" number you are attempting to send a message has reacted to your previous message with "STOP".
One solution is requesting the recipient to subscribe again by using the "START" keyword in a text to your Twilio sender. There are several other opt-in keywords also. In addition, make sure the recipient has given his consent to receiving messages from you to avoid issues.
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