Because your ADSL is installed as an overlay service on your regular telephone line, it's designed to let you talk at the same time as being connected to the network; this creates line noise.
If you want to use your ADSL phone line for voice you have to have a line filter for each phone outlet that is on the same line.
There is nothing special about them, just little pip cables with a filter box attached.
$12.00 each from these guys...
http://www.i-tech.com.au/products/item3379.asp