Each SIM card, from any telephone operator (Vodafone, TIM, tre, Wind, etc.) has an associated PIN code and PUK code, which are two security codes associated with the SIM that are delivered at the time of purchase of the SIM (or in the case of a new phone with SIM included).
THE PIN CODE. The PIN code is 4 digits and can be changed. It is asked when you turn on your phone, not every time you unlock it. If you make a mistake in dialing the PIN code 3 consecutive times, the phone will lock and to unlock it you need to enter the PUK code.
THE PUK CODE. The PUK code is 8 digits long and cannot be changed, it is unique for each SIM. Once the PIN has been blocked, the mobile phone, depending on the model, may ask you to enter only the PUK code to access the phone, or to enter the PUK code and choose a new PIN code.
Once you have entered the phone after entering the PUK code, the next time you turn it on you will be asked for the PIN code again, as the attempts are reset (if you get the PIN wrong three more times, however, you need to enter the PUK again).
If you forget the PUK code, you can find it behind the card in which the SIM card was inserted (the one in the shape of a credit card, with the PIN and PUK codes which are covered by a layer of material that must be scratched off). Some operators such as Vodafone allow those who are registered on their site to retrieve the PUK code from the personal DIY page.
WARNING: If you enter the wrong PUK code 10 consecutive times, the SIM will be blocked and you will need to replace it.
To recap: if the PIN is wrong 3 times, the phone asks for the PUK. If the PUK is correct, the phone is accessed, while if the PUK is wrong 10 times, the SIM is permanently blocked.
Those who do not want the PIN know that it can be removed: just access the SIM settings of your phone and disable the PIN request.