I don't know why SMS has become a code, but what's going on?

This estimate is caused by someone illegally setting up a signal base station near you. They make the illegal base station signal very strong, forcing your mobile phone to receive the signal from their base station. The signal may be incomplete, so what you see is the code. They illegally set up these base stations for illegal profit, such as sending you text messages by pretending to be a certain number.