Network locks are encrypted on mobile phones with a unique string based on their IMEI number. Only an unique unlock code can remove the network restriction from a mobile phone. You can get the the unique unlock code for a mobile phone either from the respective network provider or from any trusted vendor online.
For example you can visit www.Mobile-Unlocker.com to get the unlock code. This site provides accurate unlock codes for over thousands of international mobile models at affordable cost with easy unlocking instructions. Good luck mate.