Beeping when ADSL modem is plugged in on quiet line test

So for the past few months my parents have been complaining about a lot of noise being heard whenever they make phone calls. They called the BT automated helpline which said there were no issues on the line itself. I looked into the issue and the broadband is also a bit slower than it used to be (2000 kbps - 4000 kbps vs 6500 kbps). 90% of the noise goes away when I switch from the cordless DECT phones to a 30 year old corded phone, however that doesn't fix any of the other issues.

I thought maybe the filters are busted (they are around 10 years old), so I ordered some new ones (no-name brand from eBay mind) but that made no difference. My ISP instructed me to try the quiet line test which I did and heard some beeping noises. I then retried it without the ADSL modem plugged in and the line was silent.

The ADSL modem is plugged into a filter then an extension, all phones are also plugged into a filter then the socket. I tried plugging the modem and corded phone into a filter then master socket and unplugged everything else, but still had the same issues with the quiet line test.

So the issue is somewhere with the ADSL... The thing is I thought the point of filters was to separate the voice and data frequencies from interfering with each other. So I'm not sure if the issue is the modem or the filters. Any help would be appreciated.

Edited by deleted (Tue 22-Oct-13 15:12:10)

Does the master socket have the split faceplate, so that you can unscrew the bottom half and reveal the test socket?

At which point the extension wiring should stop working, and you get a real test of how the line is performing, rather than the noise pickup from what sounds like lots of extension wiring.

Filters do split the voice and data, but if a bit of wiring/kit has a dodgy joint in it, this can transpose the ADSL frequencies into the region where you can hear them, and thus the filter will do nothing to it, since it is now in the block of frequencies needed so you can hear people talk.

One trick is to put two filters in series to double filter the phone. Ebay may or may not be a good place to get filters, some good stuff on there and some very poor quality stuff too. The issue might even be the ADSL modem and a solder joint going bad due to its age, so try another ADSL modem on the line.