If you have tried 2 different filters, they aren't the right ones. Contact your provider and tell them your issues. xpcman is correct, you need a line filter on the phone line. If you are not installing them on the EACH AND EVERY PHONE THAT IS PLUGGED IN, even just one getting RING voltage will kill the synch!
Please understand, I used to deploy DSL/ISDN/POTS for a living!