*#0000# – Display the software version, the date when the software was built and the phone model. In most cases, the phone model is not the one used by marketing. It is the internal model used inside Nokia.
*#06# – Display the IMEI (International Mobile Equipment Identify) of your phone.
*#2820# (*#BTA0#) – Display the Bluetooth device address of your phone.
*#62209526# (*#MAC0WLAN) – Display the MAC address of the WLAN adapter. This is available only in the newer devices that support WLAN.
*#7370# (*#RES0#)- Reset your phone to the default factory settings. Usually you will need this code when you want to reset your phone back to the same condition when you got it for the first time. When the phone ask for security code, the default one is 12345.(if the code is wrong, you’ll have changed it)
Warning: Before you do reset the phone, make sure that you don’t have important data. Your data on the phone memory will be lost once you reset it.
*#92702689# (*#war0anty#) – Display the life timer information of your S60 phone, i.e. the total time of phone call (in minutes). Some people have asked how to reset this timer; as far as I know the only way to reset it is by reflashing the phone.
*#7780# (*#rst0*) – This will soft reset your phone. It is different from *#7370# because all data, like contacts, calendar and notes, are not removed. It will only reset the phone settings, such as profiles, themes and shortcuts. The default lock code is 12345 again.
There is another key combination which is used to hard reset the phone (similar to #*7370). It is useful in some cases, for example when the phone cannot reboot. The trick is by switching off the phone then press and hold Call key (Green key) + * key + 3 key + power button for a couple of seconds.