A phone system is made up of several telephones that are linked together to provide advanced telephony features such as call handling and transferring, conference calling, call metering and accounting, private and shared voice message boxes, and so on.
The telephone instrument, as it has been since its inception, is comprised of the following functional components: a power source, a switch hook, a dialer, a ringer, a transmitter, a receiver, and an anti-sidetone circuit. These aspects are discussed in more detail below..
Source of power
The electric current that powered the telephone circuit in the first experimental telephones was generated at the transmitter by an electromagnet activated by the speaker's voice. Because such a system could not produce enough voltage to produce audible speech in distant receivers, every transmitter since Bell's patented design has used direct current supplied by a separate power source. The first sources were batteries in telephone instruments, but since the 1890s, the current has been generated at the local switching office. The current is supplied by a two-wire circuit known as the local loop. The common voltage is 48 volts.
Switch hook
The telephone instrument is connected to the direct current supplied by the local loop via the switch hook. The receiver was hung on a hook that operated the switch by opening and closing a metal contact in early telephones. This system is still in use, though the hook has been replaced by a cradle to hold the combined handset, which houses both the receiver and the transmitter. The mechanical operation of metal contacts has been replaced in some modern electronic instruments by a system of transistor relays.
Dialer
The dialer is used to enter the phone number of the person to be called. Dialer signals activate switches in the local office, establishing a transmission path to the called party. Dialers are classified as rotary or push-button.
Ringer
By emitting an audible tone or ring, the ringer alerts the user to an incoming call. There are two kinds of ringers: mechanical and electronic. The switching office generates a 20-hertz, 75-volt alternating current that activates both types. The ringer is typically activated in two-second pulses separated by a four-second pause.
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