I've tried new phones, replaced the filters, brand new router, not using any extension cabling. Any thoughts on why this might be happening? :)
You've covered all the ground I would have suggested, klipp, call it into support and have them test the line.
Do you have any voice quality issues?
whats your stats on the line.
this normally implies a fault on internal wiring.
can you do the following?
plug ROUTER ONLY into test socket (via filter) and then ring your line and monitor for dissconnects, if it does its a BT problem (more than likely) then if it does not dissconnect your adsl plug a phone in as well (preferably wired) and repeat test, again if it does not dissconnect your adsl try again but answer phone.
if all above prove ok and no dissconnects then replace faceplate and repeat, if yuo then get dissconnects your internal wiring is at fault.
I know you have probarly done most of above but BT now gharge if the problem is past the test socket!!!