In order to hard reset a Panasonic T21, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Panasonic T21 device will be brought back to the default configuration of a brand new device.
1. Power up the phone by pressing the Power button.
2. Open Main Menu and go to Settings > Backup and Reset > Factory Data Reset.
3. Continue by selecting Reset Phone > Erase Everything.
4. Done! Your phone should now reset and restart.