Firstly, I have to say I haven’t tried this, so can’t make any personal comments. I will probably be ordering one though.
Its a little plate that fits into your BT phone socket box and provides a better way of filtering out line interference. Judging by the customer reviews, it seems to work for the majority of people.
Have a look at the I-Plate at BroadbandBuyer.co.uk.
Photo This & That 