OK I've been wondering for about 2 months how to set a specific codec on a Nokia e and n series. The only way was a OTA message but most providers are stopping this messages.
Finally I've got a solution for all nokia users that want to customize the codec and nat settings. Nokia realased a program to choose all the sip settings and it works very well!!!!
just search SIP Tools v1.1 on nokia forum.
ps: most providers firmware is blocking this program, as H3G and Vodafone so if you want to use you have to have nokia's original firmware....