The SMS function is off.

There are two possibilities. The first is that you send a lot of short messages in groups, and there are restrictions on movement. Generally speaking, as long as you send more than 200 text messages per hour, it will be regarded as spam by default, and then the function of sending text messages on your mobile phone will be turned off for you. Another possibility is that you send a message to others, and then others report you, complaining that the message you sent contains illegal or fraudulent information, and then in this way, Mobile will turn off your SMS function for you.