Here is a picture of an RJ11 jack, the original telephone quick connect:  Notice the normal telephone drop cord uses the center two pins, and which are usually the red and green wires.  Second lines in a house are usually wired on the second line, which uses the yellow and black wires.  Most houses are wired with two pair cable to provide these two lines.  This cable is rarely color coded as red/green and yellow/black however.