make sure every phone(or connection) in the house is filtered (plus the modem). If it's not DACS'd and been set up by BT it should work. If you dont have enough filters to go round just unplug the phones from the line and this should prove fruitful - barring a hardware fault of course.

Basically you should have phone > filter > line, and modem > filter > line.

ie. You cant just plug 1 filter in the main socket and expect it to work.