This may help, A single phone line uses a pair (2 wires) which starts at the interface and can loop from jack to jack or be "t-tapped" as long as the the wires dont touch together. There is no positive/negative (polarity) to worry about anymore. The Interface (box) outside may have 2 pairs of terminals(line 1 and 2). Just move the wires from the unused line to the used. Hope that helps some.