If you have a BT line and key in 17070 you can check your line in various ways.
One of the key ones is "Quiet Line" - if your line is noisy you can get BT to sort it out by ringing 151; best to speak to an engineer if you can.
Earlier this year I had problems and asked for the gain to be increased; this actually made things worse.
In the end, BT did a check from the exchange on my system; I had to gradually disconnect all the phone extensions, modem etc during the test.
It turned out that the fault was in BT's section of the line (poor earthing) and an engineer arrived the following day.
He replaced all the wiring from the house box (the one at the front of your property) to the master outlet (also replaced). The wiring was routed under the floorboards instead of under the skirting boards, making for a tidier layout.
After a full check of the replaced wiring via the exchange two hours later, he finally gave it the thumbs up, leaving me with an absolutely silent line and far superior dialup speeds.
To further emphasise BT's commitment, the engineer's boss turned up next day to check out the work which had been done and, after 20 minutes' thorough investigation, pronounced it was to his satisfaction.
The fact that the fault was in the BT section of the line from the pavement meant that I did not have to pay the normal £50 plus VAT charge.