Below are some reset codes for various Series 40, 60 and the new Series 80 Communicator series (Nokia 9300/9500):
Full Reset: (clears everything off the phone memory!! Be sure to back up your contacts!!): *#7370#, 12345 and then Yes.
Soft Reset: *#7780#, 12345 and then Yes. (Also works on Nokia S40 phones as a full reset)
*#335738#
*#delset#
This deletes the Email, GPRS, MMS settings that cannot be erased via phone options. (Newer Series 40 phones only i.e. 6230(b))
Nokia 9300 (not confirmed) and Nokia 9500 (based on link below):
1. Turn off phone and take out battery
2. Insert battery, and while it boots up press and hold Ctrl + Shift + F
3. After a few seconds you will be asked if you want to format or cancel
Other useful codes are:
*#0000# Firmware Version/Date (All Nokia GSM handsets)
*#92702689# (*#Warranty#) – Shows Life Timer, Phone Manufacturing, Repair and Purchase Dates (Series 40 phones only, you need to power off and back on after viewing this menu).
*#06# IMEI of your phone (standard to all GSM phones)