I can think of three things:

1. Most consumer-grade routers including those provided by the telco, may not have the horsepower and resources to handle all your devices. How many devices do you have on your network?
2. Check or replace the DSL filter. Do you hear modem-like sounds when using your telephone? You shouldn't be. Oftentimes, a power surge from a nearby lightning strike could could kill these filters.
3. Remove everything from the telco line and just connect your phone. Press any number (to kill the dial tone) and listen carefully for any noise. You should only hear absolute silence. If you hear static and/or humming noises, that will definitely interfere with the DSL signaling.