Metal cables are used as telephone wires because the speed of sound in them is very high. This is important for transmitting electrical signals carrying information over long distances with minimal loss and distortion. Metal cables are made of copper or aluminum, which are good conductors of electricity, and their low resistance allows the electrical signal to travel long distances without significant attenuation. The high speed of sound in metal cables also allows for quick transmission of the signal, making them ideal for telecommunications. Cost and local sourcing may be factors in choosing a specific type of metal cable, but the main reason for their use is their ability to efficiently transmit electrical signals over long distances.