What does it mean that the message has been swallowed?

SMS messages will be swallowed when the phone is shut down.

Look at the downtime, if it does not exceed 24 hours, you can receive it. Because after the text message is sent, if the phone is shut down, the text message will be retained by the operator's number center for 24 hours. When the phone is turned back on within 24 hours, the text message will be re-sent to the phone. If it exceeds 24 hours, the sent information will be automatically destroyed, and the sending will fail.