Check the DSL line connecting your modem to the wall jack. It shouldn't have a DSL filter on it. If it does, remove it.
DSL filters split the signal coming into your home between your phone and Internet. They're important, and you've probably been told to put DSL filters on all devices in your home that connect to your phone line. That's true. EXCEPT for your modem.
If you install a filter on the modem, you're stopping the DSL signal from traveling to the modem (which you don't want to do). Conversely, if you have a phone and do not install a filter, the DSL signal will interfere with your phone, causing noise and/or making your phone unusable.