I don't know for sure what that means, but I can take a guess.
They record all the calls made, and my guess is that they have a "white noise" filter on the phones so that the (often quite loud) background noise doesn't make the recording unintelligible. The public would be told that the filter was there to "improve call quality" - and it does probably do that.
I've gotten calls from guys who were on systems without the white noise filter and the conversation is nearly unintelligible on my end.
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