My line dropped 0.5db of margin when the phone was in use, until one day it went and started dropping 5.5db. This was when I was on ADSL2 with Be.
Much research and talking on forums with BT SFI engineers determined it was most likely an HR (High Resistance) fault. Good luck finding it! Took me years.
You'll probably have to do what I did ... what until it breaks down completely then report it as crackling on the voice side.
In my case, it turned out that a link wire in the PCP cabinet wasn't screwed down properly - so every time BT worked in there it was wiggling my cable against the connectors causing all sorts of intermittent fun.
Make a call, log all the stats before, during and after it. Send them to plusnet - the more data they have, the more they can help you.
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