If using DSL, a DSL Filter must be connected to every thing, phone, answering machine, faxes, etc.

Everything except the DSL Modem/router, itself.

You may share a DSL filter.

wall > Dsl Filter > splitter > multiple devices

Also cordless phones, 2.4gh, use the same frequencies as the wireless routers do. Get rid of the 2.4gh cordless phone and purchase a 5gh one.