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PUK code is the unique special key that you use to unlock the SIM card. Sometimes when you turn off the iPhone and turn it back on or replace SIM, it will ask for PUK code. It is asking you to enter it, so it can verify you are the correct owner of the SIM card. Only two people knows it. You and your carrier network. Check the papers came with your SIM and you will see the PUK code on it.
Your SIM is currently blocked and you need the PUK code to unlock it. If you don't have the papers came with your SIM, card then you have to contact your carrier network(AT&T, Verizon etc) to get the PUK Code. They will give it to you in a few minutes.
Remember, DO NOT enter incorrect PUK codes. If you enter incorrect PUK mode multiple times, you SIM card will be PERMANENTLY disabled and you will have to get a new SIM card.
Only enter the correct PUK code after getting from your carrier network and enter it on your iPhone. If you tell me who your carrier network is, then I will send the details to contact them.
If you have any questions or would like any additional information, please don't hesitate to ask.