I called him and heard him talking on the phone, and then I received an automatic reply message in less than a second, saying that he was busy!

This has something to do with mobile phones. Settings of Android operating system on mobile phone-> Call-> Mobile phone settings-> The default information is here. You can edit the default information. My mobile phone is: I'm busy now. I'll call you back later. This will automatically send this message when you press "Don't answer the phone". General caller ID can also display missed calls. If your phone number is stored in his address book (contact), your name will be displayed automatically.