Every time a mobile contract machine changes its mobile phone card, it sends a short message to its own card to deduct the fee. How to cancel?

First of all, since it is a contract machine, the buyer will have a record in the contract that it is not allowed to change the mobile phone card unilaterally. Your operation itself has violated the contract. Secondly, you can check whether there is security software in your mobile phone. Some security software has protection settings, that is, after changing the mobile phone card, the mobile phone card in your mobile phone will automatically send a message to the original card to remind you that your mobile phone is in an unsafe state. If similar software is installed in your phone, you can change the settings or uninstall the software.