Is there a default charge for incoming calls and on-hook SMS? The Xiaomi mobile phone I bought has a preset call hang-up message function. I wonder if this is a charge.

Of course, the charge is that you hang up the phone and send a message to the other party. It's just that you set it up in advance, which saves trouble. You don't have to edit the message, just send it directly. In fact, it is the same as hanging up the phone and then sending a message to the other party in the information interface.