In order to hard reset a Nokia 3120, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Nokia 3120 device will be brought back to the default configuration of a brand new device.
1. First, turn on the phone.
2. Next, press Menu button and go to Menu > Settings > Restore Factory Settings.
3. Tap in the 12345 lock code.
4. Phone will reset and restart and the hard reset process will be over. Done.