Here are some useful short cuts I had to remind myself of when I was setting up my new N97:
*#06# display IMEI of your phone.
*#0000# display firmware version and its date.
*#92702689# display "warranty info".
*#62209526# display WLAN MAC address of your phone
*#2820# display Bluetooth address of your phone.
Resetting your phone (use @ your own risk!)::icon4:
If you're trying to solve a problem use this code first! ..
*#7780# "soft reset" :sets factory defaults WITHOUT erasing your data.
Use this code if you REALLY must :icon4::icon4:
*#7370# "hard reset" : sets factory defaults AND deletes your data!
Finally there's Firmware rewrite - NOT recommended unless you really need to to change language sets or something. 😉