u only need to attach the splitter on the line from where u want the DSL connection. However, please make sure that there should be no parallel connection(s) in the same room (where u r using DSL; can have in other rooms). However, if you also want the telephone connection in the same room where u have DSL; take tht thru the "Phone" socket of the splitter.Let me explain, there are three sockets in the splitter:1. Line.2. DSL.3. Phone.Line & DSL sockets are on the same side and Phone is on the other side.Line: In this socket ur telephone cable is inserted from the phone jack.DSL: In this socket, insert the DSL cable and other end goes to the DSL modem.Phone: Insert the telephone instrument cable in it. If you want a DSL and telephone connection in the same room.This way it won't affect the speed of ur DSL connection. Plus, there will be no disturbance in ur phone line.Hope it helps !!!! 😛