I’m in the process of selling my Nokia N70 and I just found this helpful guide on Howard Forums with instructions to “Deep Reset” or master clear the phone so none of my data gets left on there for the next owner. 😉

The codes below should work on all Symbian S60 (Nokia, etc) phones to wipe them clean:

Per Nokia FAQ:
Lock Code = 1234 / Security Code = 12345

Normal Reset

(*#7780#) : Restores ini files from rom but preserves user data (photos, 3rd party apps etc)

Deep Reset

(*#7370#) : This reformats completely the C: drive. All applications and files stored on this drive will be lost and clean default files will be rewritten.

[ From HowardFormus Wiki ]