This is very interesting comment, what you are saying is you get better voice quality (MOS) with 10.2 (AMR_FR) than 12.2 ?
What about your results on codec AMR_HR ?
Are you using B10ED10 ?
Do you use TFO as well ?
If you dont mind i would like to get more info because here i find the audio is bad eventhough we are always using the 12.2 AMR_FR