half/full/enhanced rate activation?

You don't have the option to activate/deactivate like in older Nokia phones. The switching is made automatically, I think.

The phone will use whatever audio codec that the network says it will support and that the phone must use. Settings on phones will have no influence over it (even on older phones).

Despite other peoples comments, here are the keystrokes used to change codec rates (even if they dont work), i found them on some site:

Signal Processing
*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%

anarkiz, have you tried the codes? They don't work on S60.