Yea. If I connect the filter to the Modem cable the connection becomes limited for some reason and does not work. If I connect the filter to the printer cable it makes no difference, as I said before, I disconnected everything once (including the printer) and left only the modem with the router and still had interference when calling.
How old is your filter? I've seen plenty of them go bad and not work. Sometimes it's other devices that are causing the interference at which point they may all require a filter.