Call your ISP. They'll be able to test your phone line for any faults.
Also, can you get your hands on a spare/old router to check if it is the router that's causing the problem? Do you have any other issues with your phone line? (noise, crackling on the phone etc?) Have you tried connecting the router to the master socket? The master socket is normally the phone socket that is closest to your front door/ADSL line to your house.
When you call your ISP they will probably ask these questions, so it helps to know before hand.