It could be either or could also be the house telephone wiring or microfilters.
There should be a master BT socket where the main BT line comes into the house, and it should have some screws to enable removing half of the face plate. If you remove the face plate you will reveal the test socket. Plug a microfilter directly into this and then the router into it and see if it still happens. If it does then try another microfilter just to rule out the microfilter.
This test will identify the problem as the house wiring if it doesn't happen anymore, or a broken microfilter if it does and then doesn't when you change the microfilter.
If it still happens, then take the router to another working adsl house and test it there. If it ill happens then it is the router, if it doesn't happen then it is the ISP.