This method deals with the serial or imei number of the phone.
==>while the phone is not yet locked, dial *#06# to get the imei or serial number of the phone.
==>go to unlockitfree.com
==>When the page loads, select, free remote master code
==>In the area provided for imei/ serial number, insert the serial/ imei number of the phone.
==>Click on generate. Once done, a number will be generated, that's the master code for the phone, copy and keep the number.
When ever that particular phone is locked with security code and you want to unlock it, even though you don't know the phone's security code, just enter that number, i.e. Master code and it'll be unlocked!
Note:-please note that This is for educational purpose only, do not misuse it on your friends' phone.
your comments will be highly appreciated.