If it happens again try the Quiet line test
with your phone. 17070 option 2. If it crackles you need to do the same test again through the test socket inside the master,without the router connected, and see if that clears it.
If it still crackles then you report an intermittent voice fault to your phone rental company, though getting it fixed can be difficult with it being intermittent. Don't mention broadband
when reporting it as that let's them off the hook.
Even if it clears it, you need to put the setup back to normal and see if it comes back, as it might just have been coincidence. If it does come back, disconnect the router and turn it off and try the test again. And similar variations to make sure it isn't something at your end causing it.
You'll probably find your target noise margin is high now
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