I'll get shouted down by others, but in my book, the way to get this fixed is to report it as a broadband fault to your broadband ISP .... not so unreasonable considering that is what you've come here to report

An SFI engineer will understand what fault is causing the intermittent noise (an HR fault) and will have their tester in sync and be able to see the errors using the phone at the same time causes .... with this, they'll have something to chase fault wise.

Thanks for the insight Zarjaz, I think you're right, this may well be the route I need to take!

Could my extension wiring possibly be the fault? I'd disconnect it, but I have old sky multiroom boxes that need phone lines. (Grrrr) I'm trying to avoid any (further...) charges etc.

The noise comes and goes, I can hear it over the dial tone, and given the disconnects I also see, it seems quite random (middle of the night in some cases - maybe the sky boxes dialing out?).

Is it of any relevance that running a line test clears it for a while? And also, is it normal for a line test (the automated online one) to disconnect your broadband (so you never see the result!)?

Your test socket should isolate the extension wiring, give that a try.

Tried that, and didn't hear any noise, but given that it's intermittent, it's hard to say if this is eliminating the fault.

It's fine right now, so maybe they did something in the background. If it happens again I'll try without extension wiring, and if still a problem, try the BB fault route I guess.

Extension wiring is the stuff of nightmares, it basically acts as a massive aerial, pulling in interference.

The other thing you could try is an RF3 filtered faceplate, my understanding that negates the internal wiring acting as an aerial.

Next time you have the noise, how about turning off the Sky boxes and removing their power supplies from the mains sockets.

I've got the v2 faceplate, and disconnected the ring wire some time ago, so hopefully I'm as protected as I can be, whilst still having the extensions connected!

Is this known to help? I've disconnected them from the extension boxes, and no difference.

Would switching of a sky box upstairs affect a router downstairs? (The one upstairs is old, and probably close to failing!)

Just trying to get my head around how an electrical disconnection might help

A power supply should be "quiet" in an EMI (Electro-Magnetic Interference) sense. There will be noise albeit at a low level on a well designed and manufactured unit. However, it is often the case that ones designed and built cheaply or a failing PSU will create a lot of noise and that can be from Hertz right up to several MegaHertz in frequency - some will be radiated and other conducted along the attached cables. So, even if disconnected there could be transmitted EMI which a a phone line could pick up.

Thanks. Much better explained than I would have done. I just know that simply having them plugged into (live) mains can cause noise.