What's with the expired SMS?

In fact, every time I send a message, I don't send it directly to my friends.

But the number sent to the information center first (different in each region)

This number is responsible for sending it to your friends again.

General information is only stored in the information center number for 24 hours or two days.

If your friend's phone is not turned on or in service area or stops receiving these two days, the information center will keep this message for you until your friend's mobile phone is normal.

Your friend's mobile phone will be turned off for a long time, and the message will be invalid after 24 hours, that is, flag SMS.