What is the reason why there is no text message on the mobile phone?

The original function of a mobile phone is to make a phone call and send a text message. Occasionally, the mobile phone can't send a text message for many reasons.

The reason why SMS can't be sent out:

1, first confirm whether the flight mode is on, whether there is a signal, and whether there is arrears. In these three cases, you cannot send a text message;

2. If the other party's mobile phone is turned off, there is no signal, there is arrears, or the user is blacked out, the same message will not be sent;

I accidentally changed the message center number, so I can't send the message.