*#3370# - Enhanced Full Rate Codec (EFR) activation. Increase signal strength, better signal reception. It also help if u want to use GPRS and the service is not responding or too slow. Phone battery will drain faster though
Most of these worked on my 6230 back in the day. (The ones about detecting radar and setting off shop alarms and such are bogus though, clearly.)
As was stated above, most of these were for Symbian devices where the primary interface was the dialer and/or a primitive menu system. As the interface evolves I'm sure most of them will vanish. I'm actually quite surprised anything beyond *#06# works, since I doubt there was much/any cross over code from S40/S60 into Maemo. (If there were, more phone-based codes & SDDS would have worked from the get go.)
The main reason for having *#06# work is it's usually the first thing a Nokia rep will ask for. With the IMEI code they can find the model/build and pick the right index card to start trouble shooting with.
some of those codes are officially part of the GSM standard
edit: btw, becarefull with some of those codes, specially the longer ones, you might endup setting your line to be used (credits included) by someone else
