To keep unauthorized usage of you cell phone from happening, you can set a 3 – 5 character PIN (Personal Identification Number) number on your mobile/cell phone, which will act as the password for accessing sensitive information on your cell phone, and allow access to only those who have the privilege.
You risk locking your phone/SIM card if you enter wrong PIN number three times. If in case you lock your SIM card, then to reopen the lock you’ll have to enter PUK code which will gain back access to your SIM card. Again if you enter wrong PUK code three times, you risk blocking the SIM card completely and the only option for you will be to walk up to your nearest Airtel shop and replace your old SIM card with a new one.
PUK code is a unique code for each SIM card and will be provided by your service provider. To find the PUK number of your cell phone, send this message PUK <PhoneNumber> to 121. Ofcourse, if your cell phone is locked send the message from another number (but it has to be another Airtel number) and you’ll get your PUK number as a reply message on the other number.
Enter the PUK number in your cell phone and you’ll be able to access your cell phone as before.
**Update** Bala S Left this comment, which is very helpful for those who still are not able to figure out how to find PUK number of Airtl Sim card:
Just type your mobile number and sent to 785 (EX; 9894111100 to 785)
Shortly u will receive ur PUK code in ur mobile message inbox airtel sim blocked solution.
Default Pin no is: 1234 and 0000
Related reading: Send Free SMS in India, US and Canada