I came across Nokia Universal Codes - Nokia

Nokia Phone Codes:

01) *3370# Activate Enhanced Full Rate Codec (EFR) - Your phone uses the best sound
quality but talk time is reduced my approx. 5%

02) #3370# Deactivate Enhanced Full Rate Codec (EFR) OR *3370# ( Favourite )

03) *#4720# Activate Half Rate Codec - Your phone uses a lower quality sound but you
should gain approx 30% more Talk Time.

04) *#4720# Deactivate Half Rate Codec.

05) *#0000# Displays your phones software version, 1st Line : Software Version, 2nd
Line : Software Release Date, 3rd Line : Compression Type. ( Favourite )

06) *#9999# Phones software version if *#0000# does not work.

07) *#06# For checking the International Mobile Equipment Identity (IMEI Number). (
Favourite )

08) #pw+1234567890+1# Provider Lock Status. (use the "*" button to obtain the "p,w"
and "+" symbols).

09) #pw+1234567890+2# Network Lock Status. (use the "*" button to obtain the "p,w"
and "+" symbols).