QCN is a file for Qualcomm models to store mobile phone IMEI, MEID and other network calibration value data, and it is located in the EFS partition of the mobile phone. If the QCN data is damaged due to some accidental operations, the SIM card may not be recognized due to the loss of the mobile phone IMEI, and problems such as no signal of the mobile phone, inability to surf the Internet and make calls may occur.

At this time, if there is a QCN backup file, it can be solved by restoring the backup. Even the problem that the phone NV data is corrupted may be solved by restoring the QCN.
This video will teach you how to backup QCN files

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