you should perform a MS to MS test, and see when is the VQ degraded. I guess it has to do with the codec being used.
If you find out (or assume) that TFO+AMR is the couple that leads to bad VQ –> You can set up the TFO so that it is only activated when legacy EFR or HR is used, and not when AMR is used.
If you search “TFO” within the topics of this forum, you should find a post with all parameters listed and all main explanations about TFO parameters.
But the first step is to find out the codecs which are “faulty”.