What's wrong with SMS not showing that it has been sent?

It was not sent successfully. When the mobile phone network signal is not good, sending information fails and "delivered" will not be displayed. During this period, the mobile phone is not in the service area, there is no signal, and you can't send text messages.

Demonstration model: Iphone 12 System version: iOS 14 The receiving time of the other party's mobile phone is short, and the validity period is 1 day. If the other party's mobile phone is turned off or out of service area, and there is no signal during this period, you can't receive SMS.

The other party is not online. If the receiver is not online or the mobile phone network is not turned on, the short message will not be sent, and the short message status will be displayed as sent instead of sent.

Generally speaking, when the other party's mobile phone has a signal, or turns on or turns off the flight mode, the other party receives the message, and the user who sends the message will receive the notice that the other party has received. In addition, even if the other party has blacked out and the other party receives the short message, the blacked-out person still shows that the message has been delivered here.