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Have you tried a different splitter and/or filter on this jack ?
Filters would be between the wall jack and the Non DSL Device (Phone, Answering Machine, Fax, etc).
It sounded like you were using 1 jack for both the DSL Modem and a Telephone, which would require some type of spitter. One end would go directly to the DSL Modem and the other end would have a filter between the splitter and the phone.