Mobiles Signal Processing Codes
*3370# - Enchanced full Rate Codec (EFR) activation. Automatic restart can be seen.
#3370# - Enchanced full Rate Codec (EFR) deactivation
*4720# - Half Rate Codec activation. Automatic restart can be seen.
#4720# - Half Rate Codec deactivation
Enchanced Full Rate will give you much better sound quality when you enable it.