To erase all of your Contacts at once:
1. Press Menu > 6 > 4
2. Enter the four-digit lock code.
3. Press 5 [Erase Contacts]
4. Select Erase all #s then press Ok

To reset your phone to the factory default settings:
1. Press Menu > 6 > 4
2. Enter the four-digit lock code.
3. Press 6 [Reset Default]
4. Press Ok, select Revert then press Ok.