itself is a rather simple appliance. A microphone, called the transmitter, and an earphone, called the receiver, are contained in the handset. The microphone converts speech into its direct electrical analog, which is transmitted as an electrical signal; the earphone converts received electrical signals back to sound. The switch hook determines whether current flows to the telephone, thereby signaling the central office that the telephone is in use. The ringer responds to a signal sent by the central office that causes the telephone to ring. ....