Unblocking Nokia SIM card requires PUK (personal unblocking key) that only the mobile network operator can provide. PUK should not be entered incorrectly, or else you just destroyed the SIM card and you will need to request a SIM card replacement.
Nokia SIM Card and Network Provider Codes
The Subscriber Identity Module (SIM) card stores a service-subscriber key from mobile network providers. It also contains the list of services of the mobile network for the subscriber, and the personal identification number (PIN), personal identification number 2 (PIN2) and PIN Unlock Key (PUK) codes.
Nokia customers who need to change settings in a Nokia device that is related to services provided by the mobile network operators are required to enter the PIN code. The PIN code is usually 4 to 8 characters only. If you have forgotten the PIN code or have entered a wrong PIN code 3 times or more, the SIM card will be blocked. Unblocking Nokia SIM card requires entering the PUK. The PUK consists of 8 digits only. Find out how you can get the PUK to unblock the SIM card.
Note that the PIN, PIN2 and PUK codes are not provided by Nokia and are not the default 12345 security code to guard or lock a Nokia device to prevent anyone using or browsing that Nokia device.
How to Unblock a Nokia SIM card
There are two methods in unblocking Nokia SIM card:
Over the air - Enter the service keys by the network provider and request it via air. Example is the screenshots below where after I entered the service request key to Maxis network, it displayed the options on how to retrieve the PUK of the SIM card: In the above image, I simply entered *100# keys to mobile operator Maxis in Malaysia. The option on how to retrieve the PUK is displayed next: After entering my mobile number, the PUK code is displayed and a new PIN code request option that can be used in unblocking Nokia SIM card is provided.
Over the phone - If you are not familiar with how to request services to network operator via air, simply contact the mobile network provider of the SIM card. Inform the customer service representative that the SIM card is now locked, and you need their assistance unblocking the SIM card in your Nokia smartphone. They should provide the PUK after verifications of your mobile phone number. This is to ensure that you are the rightful subscriber and not any other person.
Note: If you are using a dual SIM card, make sure that you've entered the correct PIN or PUK codes to the correct SIM card.
SIM card self-destructs if wrong PUK is entered
You need to be careful in entering the PUK code because unlike the PIN code that simply blocks the SIM card when the wrong PIN codes are entered. The SIM card will self-destruct, if you’ve entered the wrong PUK code, 3 times. The mobile provider will need to replace the SIM card, if the PUK code is entered incorrectly. You will need to visit the nearest office or branch of your mobile operator to get a new SIM card.
There are known incidents that the phone will display messages such as “SIM card rejected" or "SIM card unaccepted". And this is either because the Nokia SIM card is not aligned, you changed service providers, or the PUK was entered incorrectly. If the cause is the latter, you need to contact the mobile operator for a replacement Nokia SIM card.
Image credits: Screenshots taken by the author, Nokia logo http://commons.wikimedia.org/wiki/File:Nokian_logo.svg By Z10x at en.wikipedia (Own work) [Public domain], via Wikimedia Commons