There are several stages a SMS message goes through before it is delivered to the customer. You can find the current status of a SMS message by going to Admin > Channels > Text > History. This article goes through each status and what they mean.
accepted - When a request is first sent to Twilio through an API request to create a new text message, this is the first response received by the client. Twilio will now try to associate the correct ‘From’ phone number for the message.
queued - Once Twilio finds the correct ‘From’ phone number, the message is now queued to be sent out.
sending - Twilio will now start the process of sending out your message to the nearest upstream carrier in the SMS network.
sent - The message was successfully sent to the nearest upstream carrier and has been accepted for delivery.
delivered - Twilio has received confirmation from the upstream carrier that the message was delivered to the recipient.
failed - The upstream carrier did not accept the message from Twilio. Sometimes there will be error codes in monitor’s call logs or in Twilio’s console and descriptions of each code can be found here.
received - Each incoming message to your Zendesk Text number will have a received status associated with it.
undelivered - The message was not delivered and can be caused by a variety of reasons listed here.