PUK is a code required to unlock a GSM SIM card for your mobile phone that has disabled itself after entering the incorrect PIN code 3-5 times in a row. if an incorrect PUK code is entered more than five times the SIM card will permanently block or disable itself. A method of unblocking is either contacting your service provider or use the P.U.K number usually found on the sim card holder/package.
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PUK stands for Personal Unblocking Code. This acts as a security feature on your mobile. To protect you in the event of theft, the SIM card is immediately locked if someone keys in three different incorrect PIN codes in succession.