That's not quite correct as they block the DSL frequencies from the phone side of the filter - ie a low pass filter. The DSL side of the filter is a straight through connection in all the ones I've seen.
Right, its more accurate to say they are designed to ONLY let through the voice frequencies to the telephone socket, so whatever range outside of that exists on the line is of no importance.
They exist purely to avoid us hearing the DSL tones on the phone and to avoid any rogue tones that might be in the DSL range (RFI from the phone or clicks from connecting/disconnecting the line) to feedback and cause DSL to drop.
