Why do some people have a red exclamation mark next to their text messages?

This does not mean that the transmission was successful. Generally, there is no signal, or there is arrears.

Or the number of the SMS center is wrong, so the SMS function cannot be used normally.

How to set the number of SMS center:

SMS interface, press the task key to pop up "SMS Settings";

Click [Advanced Settings] and find [Set SMS Center Number];

Enter the number of the local SMS center, and return after confirmation.