The basics of wiring a home telephone are the system requires two wires, although telephone wire contains four wires inside the cable. Most home systems use the red and green wires for the low voltage phone signal that both rings the telephone bell and carries voice or digital signals. The yellow and black wires serve as spares. In homes with two phone lines, the yellow and black wires sometimes serve as the second line.
By knowing the wiring pattern, the homeowner can easily tap into the phone system to run an extension line to other rooms in the house. Phone jacks and additional wire are available at most home improvement stores and are inexpensive. Extending the line requires removing the existing phone jack from the outlet box, connecting the wire and routing it to the new location. The owner then connects the wire to the new surface-mounted jack and attaches it to the wall. Using it requires plugging a phone into the jack.
Telephone wire is small so installers often attach it to the top of baseboards using staples. It is low enough in voltage that it presents little danger left exposed. In new construction, contractors add the lines before the installation of drywall, so they remain hidden within the walls.