For FTTC Ranges A and B, the term "Clean" relates to a line which is free from any wiring issues (e.g. Bridge Taps) and/or Copper line conditions, and the term "Impacted" relates to a line which may have wiring issues (e.g. Bridge Taps) and/or Copper line conditions.
Throughput/download speeds will be less than line rates and can be affected by a number of factors within and external to BT's network, Communication Providers' networks and within customer premises.
Note the reference to "Copper Line Conditions" (which although not stated, also includes Aluminium - poorer conductor).
And also "within the customer premises" - ie from the Network Terminating Box (NTE) onwards to any and all devices both directly and obviously connected to your internal phone extensions; and also other devices such as plasma TV sets, micro-wave cookers etc.
If you have not already done so recently, you should go through the Quiet Line" Test procedures, if available.
First try dialling 17070.
If you get an automated reply about testing etc, press 2 for the Quiet Test - 4 if SKY is your Phone Provider.
Apart from a very low hum on some lines, the line should be quiet, NO loud noises, "snap, crackle and pops" etc.
Having established that you have access to the QLT, then go to the NTE, the generally white box where your phone line becomes available in your house.
If it is the more recent type, remove the front section to reveal a single Test Socket.
Otherwise plug in preferably a corded, (preferably older and siimple) phone.
Repeat the QLT - if you hear any untoward noise, report this to your PHONE PROVIDER - who should then deal with BT including Openreach - the latter being the owner and maintainer of most phone lines in the UK.
Repeat the QLT withan ADSL Splitter added, ie NTE through Splitter to phone.
You may also wish to test your own phone wiring within your house - your responsibility.
Reconnect at the NTE, then progressively connect individual extensions and devices, doing a QLT each time. No Charge!
Also progressively switch on other devices, such as TV sets, micro-wave cookers, alarms etc, again doing a QLT at each stage.
Yes, it can be time-consuming - but if you are to get the best performance out of Broadband, whether ADSL, VDSL etc, delivered over the phone line, you need to work through all of those stages.