Each phone line (i.e. each phone number) is an electrical circuit requiring two wires, just like a hi-fi speaker or a light bulb or bicycle horn.

In some areas, fiber optics or other new technology eliminates the need for a complete copper path of wire stretching all the way from the phone company's central office to your phone, but you can assume that almost every phone is connected to between several feet and several miles of low-tech copper wire.

The two wires carrying a phone circuit are generally considered to be a "pair," and are usually twisted together and joined with one or more other pairs, inside an outer protective plastic jacket, to make up a multi-pair wire or cable. Multi-pair wires are typically available with two, three, four, six, or twelve pairs. Beyond twelve pairs, as the wire bundle get thicker, it is generally referred to as a cable, rather than a wire.

"Cable" sounds more impressive than "wire," but they often mean the same thing. There is no fixed point where wire gets big enough to be called cable. A big boat is a ship, and big wire is cable.

The wire running around inside the walls of most homes and offices has from four to twelve conductors, often arranged as two to six pairs, with each pair twisted together.