It is a one line plug. The RJ14 uses four wires and is used to handle two lines. It is commonly used for two line phones. The first step to wiring your phone system is to figure out what the telephone company has left you to work with. They have probably either left you a punch down block or a network interface box.
Most new installations consist of a network interface box.
Wiring From the NI you want to plan how you want the wiring in your location to be. The star method or homerun method is the most common method of wiring. Each extension is run directly from the NI or phone system. The other type of wiring is series or loop. In this method one long wire links all of the extensions in a series.