Here is some secret code for Nokia Phone I got, maybe it's useful for you....

CODES and DESCRIPTIONS
*3370# Activate Enhanced Full Rate Codec (EFR) - Your phone uses the best sound quality but talk time is reduced my approx. 5%
#3370# Deactivate Enhanced Full Rate Codec (EFR)
*#4720# Activate Half Rate Codec - Your phone uses a lower quality sound but you should gain approx 30% more Talk Time
*#4720# Deactivate Half Rate Codec
*#0000# Displays your phones software version, 1st Line : Software Version, 2nd Line : Software Release Date, 3rd Line : Compression Type
*#9999# Phones software version if *#0000# does not work
*#06# For checking the International Mobile Equipment Identity (IMEI Number)
#pw+1234567890+1# Provider Lock Status. (use the "*" button to obtain the "p,w" and "+" symbols)
#pw+1234567890+2# Network Lock Status. (use the "*" button to obtain the "p,w" and "+" symbols)
#pw+1234567890+3# Country Lock Status. (use the "*" button to obtain the "p,w" and "+" symbols)
#pw+1234567890+4# SIM Card Lock Status. (use the "*" button to obtain the "p,w" and "+" symbols)
*#147# (vodafone) this lets you know who called you last
*#1471# Last call (Only vodofone)
*#21# Allows you to check the number that "All Calls" are diverted to
*#2640# Displays security code in use
*#30# Lets you see the private number
*#43# Allows you to check the "Call Waiting" status of your phone.
*#61# Allows you to check the number that "On No Reply" calls are diverted to
*#62# Allows you to check the number that "Divert If Unreachable (no service)" calls are diverted to
*#67# Allows you to check the number that "On Busy Calls" are diverted to
*#67705646# Removes operator logo on 3310 & 3330