SIM Pin is password which lets a mobile phone user access information of the SIM card. This password can be of 4 or 8 digits. These PINs are just like ATM pins and users have to authenticate themselves as true owners of their cards before all the information can be accessed.
Sometimes, SIM card come with PIN out-of-the-box. But mostly, the service providers let their customers choose their own SIM pin after the intitial registration formalities are done. If the SIM card comes with PIN then the customer can change it to a number of their choice. In some cases, the SIM PIN can be changed from the phone and sometimes, the user needs to call the customer service for changing it.
Some SIM cards come with PUK or PIN Unblocking Key. If the user forgets his PIN, then he is required to put this PUK number inside for retrieving the PIN. If the PUK number is entered incorrectly for a specific number of times, then the SIM card blocks itself for ever. If this happens, the user needs to buy new SIM card. On the other hand, if the user enters the PUK number correctly, then he will get his PIN.
The user should choose a secure PIN for keeping all the information inside confidential and safe. Users are advised to choose a PIN that is easy to recall for him and difficult to guess for others. Users shouldn’t use PINs of credit card or ATM cards for security purpose. Also, it is recommended that they don’t use last 4 digits of the Social Security numbers.
Customer service representatives ask users to provide SIM Pin before replying to any questions about their account. This might seem unnecessary but it is essential for security purpose.