Do not filter the modem- as jimbopalmer said, the filters are to keep the phone frequency from interfering with the dsl frequency, not the other way around. The filters should be on anything that is connected to that phone line including fax machines, dial-up modems, satellite receivers, answering machines, caller-ID boxes, and -if still on the same phone line- on the premise including the shop, barn, garage, basement, outhouse, etc, but NOT the dsl!
The test continues...