Open iTunes on your PC or MAC.
Insert any non-accepted SIM card into your iPhone.
Connect iPhone to your PC or MAC using USB cable.
Restore iPhone via iTunes from your latest backup, then wait until data is fully restore.
Disconnect phone from the USB cable, then restart your iPhone.
Reconnect to iTunes and you will get a message.
Turn OFF mobile phone.
Insert any non-accepted SIM card into your phone.
Turn ON mobile phone.
Wait for device to promt for SIM unlock pin.
Enter mobile unlock code.
Your device will then display "Network unlock successful".