I have same problem.
I thought that Gain is problem. But setting up to the lowest value (-12) didn't make any difference and also did not lower gain (Tx or Rx). It was still loud!!!
I also thought that i missed DSCP settings for RTP, but no...
On the end I managed to get clear sound on one end, but now I do not hear other end or i hear every other word and even that word is cut on half.
Very annoing