*3370# - Enchanced full Rate Codec (EFR) activation. It will automatically restart.
#3370# - Enchanced full Rate Codec (EFR) deactivation
*4720# - Half Rate Codec activation. It will automatically restart.
#4720# - Half Rate Codec deactivation
Enchanced Full Rate will give you much better sound quality when you enable it. The new Enhanced Full Rate CODEC adopted by GSM

uses the ASELP (AlgebraicCode Excitation Linear Prediction) compression technology. This technology allows for much great voice

quality in the same number of bits as the older Full Rate CODEC. The older technology was called LPC-RPE (Linear Prediction Coding

with Regular Pulse Excitation). Both operate at 13 kilobits.(but you take up more space on the network, so they can charge you more) -

Talk-time is reduced with about 5%