Some prepaid SIM cards have a security mechanism in place that requires the subscriber to enter in a PIN code every time the GSM mobile phone is turned on. When a PIN code has been assigned it is important not to forget this code. If you enter the PIN code incorrectly into the phone 3 times in a row then your GSM cell phone will display a Blocked message on its LCD. When this occurs you will then be required to unblock your phone using the PUK code. All phones have a different set of procedures to unblock and you will need to consult your GSM mobile phone manual when this occurs. Do not simply enter in the PUK code as this method never works.