In order to hard reset a Nokia 3109 classic, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Nokia 3109 classic device will be brought back to the default configuration of a brand new device.
1. Use Power button to turn on the phone.
2. Now open Menu and select Settings and Restore Factory Settings.
3. You will be asked for the security code. Provide it. Default code is 12345.
4. Your phone will reset & restart. Well done!