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The Easy way is via the settings menu.
Goto Main menu then settings, under General Tab you will scroll down and see Master Reset.
Under Master reset you got two options. Reset Settings and Reset Everything.
You can opt Reset Everything. If Asks you code then the default is 0000.
Note: That will remove all data including Contacts, so better make a backup before.
The other way, and the most efficient way is to update/flash the Phone's Software/Firmware.
This is recommended if your Phone is getting slow, giving errors or some unexpected responses.
Firmware Flashing is easy. But you need your USB data cable for that. It comes with a new W705 within the box.
If you have it, just go to this link
. Download and install the application.
Its easy Walk-through Wizard.
I Hope you will get your issues resolved.
Feel free to ask anything related to your query.