I call someone, someone turns it off, someone turns it on, and there is a text message on my mobile phone to inform others that it has been turned on. What kind of business is this?

Dear, the business you are talking about should be the startup reminder business. Boot reminder service is a new type of call reminder service for incoming calls. Simply put, when you call someone, if the other person's mobile phone is turned off, the system will record the call; When the other party turns on the phone, the system will send you a prompt to tell you that the other party you are calling has turned on the phone or returned to the service area, so you can get in touch with him or her.