We see a few cases on this every now and then. What version of TSP on Unity and Call Manager are you using? What are you using as a voice gateway? Have you tinkered with your input gain and output attenuation settings on the gateway?
How often does this happen? Can you reproduce it? Does this happen during regular phone conversations? Have you taken a sniffer trace of a bad call? There are audio tools out there that will allow you to strip the audio stream out of a sniffer trace and listen to it.
For an immediate workaround, make sure AGC is enabled and run the SetVolume utility. That will normalize the incoming audio.
If this "started to happen suddenly" a month ago, then think back to what changed around then. Did you upgrade the call manager or unity server? Swap out a voice gateway? Any network changes? More phones? Anything? Stuff like this doesn't start happening on its own.
We have an audio troubleshooting guide I recommend you read through.
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