your ISP will be responsible, if its a BT problem they will have to speak to BT.

a line attenuation like that indicates a line distance of about 4 miles. so either your line is bad or you are unlucky enough to have the cabling go right rounfd the houses!

to eliminate a problem with your own cabling plug your router into the test socket. this is the main BT phone socket, the one that has a removeable bottom half. unscrew the bottom half and plug the router into the socket behind the plate (it'll be obvious) then redo your tests and post up uyour line attenuation.

also get bt to do a quick free line test here:
http://www.bt.com/consumerFaultTracking/

also do a quiet line test by dialing 17070 so see if you have any audiable interference on the line