Nokia phones have several secret
One that works on all phones is the *#06# which will show you your IMEI (serial) number.
Another one is *#92702689# will also shows your serial number, month and year of manufacture, purchasing date (which you can set yourself if your dealer hasn't; actually all phones I've seen has this unset. Beware, you can only ever set this once. I set mine to january, 2020.) You can also see last time of repair, and there's a scary menu item called transfer user data which I believe is some sort of memory dump or flash option or something. To get out of the menu you have to reboot your cellular.
If you have a serial cable, programs such as gnokii can enable a hidden menu called Net Monitor which will show you tons and tons of useless debugging information. Some of the information will tell you the temperature of your cellurar, the distance to the nearest base station etc. Perhaps you can triangulate this stuff to get some sort of poor man's GPS.