A PUK is a Personal Unblocking Key and it is used to unblock your phone, typically when a SIM PIN has been input incorrectly too many times.
You can’t use a blocked SIM until the right PUK code has been keyed into the device.
Some people confuse a PUK code with a PAC code – a PAC code is what you need to get from your network if you want to transfer your mobile number to another network.
You will have to contact your network operator for a PUK code and confirm your details. Depending on your network, you may be able to get your PUK code online by logging into your customer account.
Some networks include the PUK code in your SIM card’s documentation, so it’s useful to keep hold of it in case your SIM ever becomes blocked.
If a PUK code is input incorrectly 10 times, the SIM card will be permanently erased and you’ll need to order a replacement from your network operator, so don’t try to guess it.