You need an ADSL filter on everything connected to the BT master socket except the modem
Additionally the modem must have a cable of its own. If there is a telephone connected via the same cable you will get the modem disconnected when the phone rings
ADSL filters do wear out and there are many different qualities which is part of the reason why prices vary.
But the best way to avoid trouble is to buy a faceplate for the master socket which has two sockets. One for your modem, and one for everything else. This will isolate the modem from all the house telephone wiring and solve any potential problems. It also makes the use of ADSL filters unnecessary