We can definitely provide you with some more information about the unlock process. The device unlock code that you received from AT&T is used to unlock your device from the AT&T network, so it is the same as the network unlock code. When you go to complete the unlock process and enter your unlock code in your device, please follow the instructions that you were provided via email along with your unlock code, and be sure to insert the correct SIM card type for your Samsung Galaxy J3 as well.
If you're still unable to unlock your device with the unlock code that we've provided for you, it's usually due to the unlock code being entered incorrectly too many times. The incorrect entry of an unlock code too many times during the life of the device disables the unlock ability and permanently locks the device to the network. The number of attempts to unlock the device varies by manufacturer and model.
The only other reason that you could be having trouble with completing the unlock process would be if your cell phone was previously rooted or jail broken. If this is the case, you can try to Perform a factory reset to resolve the issue, but there is still no guarantee that it will work.
Please let us know if you have any further questions or concerns.
Sean, AT&T Community Specialist
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*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.