The telephone line is the incoming line of the telephone, which can only be connected to the telephone. It is divided into 2 cores and 4 cores. There are three kinds of conductor materials: copper clad steel, copper clad aluminum and all copper. All copper has the best conductor effect. Telephone line shall be used for communication engineering wiring, connection between indoor telephone communication cable system wiring, trunk line between voice communication systems, program-controlled switch, telephone.