As Ian suggested, you may want to include ALU TAC on the problem.
Why are you saying TFO does not support HR 5.15 ?
Your Transcoder configuration should include the HR 5.15 in the list of possible AMR codecs. I have never been in the TC myself, but that’s worth a check.
If it’s not done this way, then TFO will be de-activated everytime the call moves to AMR HR 5.15. I believe that frequent TFO – non-TFO changes will lead to voice degradation.