The personal unblocking key or PUK code is an eight-digit code required to unlock a Nokia phone with a blocked SIM card. PUK codes are generally supplied with the SIM card. PUK codes can also be retrieved from local service providers, according to Microsoft.
A blocked SIM card results from three incorrect attempts at entering a phone’s PIN code. Nokia phones with blocked SIM cards usually flash the message “SIM card blocked – Enter PUK” on their screens, states the website O2. Providing an incorrect PUK code too many times can lead to a permanently blocked SIM card. Several service providers, such as AT&T, offer their clients online access to PUK codes.