Filters is it. Some DSL's run a seperate line from your phone network connector and the filter is in that box, making the line running to all phones filtered in one spot. This is the old way, and the way usually used IF the phone company is your DLS company and does the install. In most other places, there is no seperate line and every place you put a phone, you need a filter. None where the DSL modem is plugged in.
I'm lucky, my phone company's main switch is about 1/2 mile away and they offer DSL. I've had it for several years with no problem.