If you have a Nokia 6300 and it is network branded, which means the network operator has locked it to their network only and has there branding on the mobile phone housing and on the firmware, you can't use a different network sim card in the mobile phone. This makes its frustrating because the sim card that you want to use might have better deals going on, so the only way around this is to unlock the Nokia 6300 using a unlock code from a generator/calculator or using special software and tools to obtain the unlock, but there is one downfall, which is you have to pay for either one. However, the Nokia 6300 is a very old phone and there is a free method that is been discovered to unlock the phone yourself at home, without paying somebody to do it for you. It works for all locked networks on the mobile phone, including o2, orange, t-mobile, virgin and Vodafone.
Instructions on how to unlock the Nokia 6300 to all networks, works on any firmware version, without using a code:
1) Remove the battery lid from back of the device and remove the battery, leave the battery out for 30 seconds.
2) Replace the battery and hold the three buttons, they are: Green call key + # + the up on the volume button.
3) Make sure you hold each button after each other firmly down and press down the power button, this is to power on the mobile phone.
4) Let go off the buttons, when you see the Nokia hand shake, then the screen will display a message saying to remove the network lock, then press OK, which is the middle button.
5) Then the screen will flash on and off for about 10 seconds and the unlock procedure is done.
6) Test the unlock, this is by inserting a different network provider sim card into the mobile phone and it will now accept it.