How can someone else's information appear when I change my number?

The question is not detailed enough. 1. Now, by default, information is stored on the mobile phone. If this happens to your new mobile phone, then this mobile phone is used by someone else. 2. Today's mobile phones have synchronization function and cloud function. Maybe someone logged in Xiaomi account, Meizu account, apple id and other accounts on your mobile phone. The network will synchronize automatically.