Hello Niex,
Try to generate .xml config file of your onu, find in it line (around 425):
<X_HW_Extend EchoCancellationEnable="1" SilenceSuppression="0" DefaultCodec="G.711ALaw" DefaultPktLen="20" PlcFlag="0" PktLostThreshold="400">
Check and change any of parameteres to around what you want. I think it may be what you need.
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