How to select/change bitrate for AMR codec

[tesk_terrus@xxxxxxxxxxxxxxxxx (Yuri Maliy)]

Hi All, i want to use AMR codec with pjsip for Symbian VAS. 
I select it with this code, and it's works fine:  
aud_param.flags = PJMEDIA_AUD_DEV_CAP_EXT_FORMAT;
aud_param.ext_fmt.id = PJMEDIA_FORMAT_AMR;
aud_param.ext_fmt.bitrate = 4750;
aud_param.ext_fmt.vad = 0;

But when i changing bitrate to another values it seems that nothing happens. 
Please help me how to change and how to check that changes have some effect.