Check for the filter. Try installing a different filter. And check every phone jack at home. If there are devices connected on the same phone line, install filter on it.
Check the dsl light on your modem. If it is flashing green/red, it means that the modem is trying to get the dsl signal from the CO. Try calling AT&T technical support, it might be a line problem then.
1-800-722-3755 24/7 technical support