I am using a free account. I am using SMS service for testing purposes. I got an number through which the messages were getting delivered successfully until recently. Since three days the messages are not getting delivered and the status of the message remains "sent" and not delivered. What could be the issue?
Sometimes, if the delivery record cannot be received from carrier partner for some reason, the delivery status remains as "sent" . You can refer to the details in this FAQ.
You may also experience limitation of sending SMS based upon location for example, in India, all SMS messages sent from Twilio phone numbers are categorized as “Promotional” , even if your message content is transactional.