Why is there a text message when my mobile phone is not turned off, if there is an incoming call?

That's a call reminder. Call reminder service is extra.

"Call reminder" service refers to the missed call reminder service provided for calling users. When the called user is busy, turned off or unable to connect, the caller is connected to the missed call service system, and according to the caller's identity and the caller's choice, the called party is informed of the incoming call information when the called party can arrive, so as to ensure that the caller's information about incoming calls is not lost and give users a better experience.

China Mobile, China Telecom and China Unicom all have call reminder services. Mobile service is called call reminder, Unicom is called missed call reminder, and telecom is called communication assistant.

Service activation: mobile users send "BLTXZS" to 1 0086 to apply for activation. After the system confirms the success of the operation, the user will receive a customized confirmation message, and there is no charge for canceling the subscription within 72 hours.

Cancel service: Mobile users send "QXTXZS" to 1 0086 to cancel service.