If Opus audio codec is used, try adding Opus formats: add opus_formats to WCS_HOME/conf/flashphoner.properties. E.g.,
opus_formats=maxplaybackrate=16000,sprop-maxcapturerate=16000,maxaveragebitrate=20000,stereo=1,useinbandfec=1,usedtx=0
If G.711 audio codec was used - for better quality use...