In order to receive message delivery information, please make sure you are sending messages by making API requests to our '/Message' resource URI. Our '/SMS/Message' resource has been deprecated and this particular feature is not available with it.
When possible, Twilio will provide as much detailed information about the delivery of your message. In most cases, this means the message will be updated with the status of 'delivered' or 'undelivered' after it has been 'sent'. There may be occurrences where this information is not available. If this happens, the status of the message will remain as 'sent'.