We have added some useful codes that may be helpful.

All models- *#06# (IMEI) -  For checking the "International Mobile Equipment Identity".

NOKIA- 12345 This is the default security code.
NOKIA- *#0000# Displays your phones software version, 
                               1st Line : Software Version, 
                               2nd Line : Software Release Date, 
                               3rd Line : Compression Type
NOKIA- *#9999# -  Phones software version if *#0000# does not work.
NOKIA- *#67705646# -     Removes operator logo on 3310 & 3330
NOKIA- *#73# -   Reset phone timers and game scores
NOKIA- *#746025625#-    Displays the SIM Clock status, if your phone supports this power saving feature “SIM Clock Stop Allowed”, it means you will get the best standby time possible
NOKIA- press and hold #-    Lets you switch between lines

SONY ERICSSON-  >*<<*<* -   Lets you find make of model, version, testing, sim lock status

MOTOROLA- Hold # key- lets you toggle between "no ring tone" and "ringer".