cyril7: Hi, dont think ther is any issue with your lan side, its the phone line side, what other phone/fax/sky boxs are on the line elsewhere in the house, have they got filters. The modem itself does not need a filter only phone devices, but leave the filter there if you must it should not effect the speed, but unfiltered phone/voice devices will.
Is there a monitored alarm (monitored or not if its across the line it can stuff DSL).
2 Cord less phones, and all have filters on. And the broadband is plugged in to the ADSL part of the filter will the phone in the other plug part.