Samsung Galaxy S21 Ultra
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It's not possible to start my phone
Solution 4 of 7: Your phone is locked to another operator
If your phone is SIM locked to another operator and you try to use a SIM from your operator, the display will tell that your SIM is invalid.
Before you can use a SIM from your operator, you need to unlock your phone using an unlock code. To get the unlock code, contact the operator that locked your phone.