In order to hard reset a Siemens C35i, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Siemens C35i device will be brought back to the default configuration of a brand new device.
1. Turn on the phone.
2. Tap in the *#9999# code.
3. Wait for your phone to reset'n'restart. Done.