being used for a phone call A typical phone call using a traditional phone is placed by picking the phone handset up off the base. The caller then
rotary dials or presses buttons for the phone number needed to complete the call, and the call is routed to the phone which has that number. The second phone makes a ringing noise to alert its owner. The first user can hold the handset so that the hearing end is next to their ear and the speaking end is within range of their mouth. They then hear a ringing noise in the earpiece. If the second phone is picked up, then the two users can talk to one another. If the phone is not picked up, the first user continues to hear a ringing noise until they hang up their own phone. In addition to the traditional method of placing a telephone call, new technologies allow different methods for initiating a telephone call, such as
voice dialing.
Voice over IP technology allows calls to be made through a
PC, using a service like
Skype. Other services, such as toll-free dial-around, enable callers to initiate a telephone call through a third party without exchanging phone numbers. Originally, no phone calls could be made without first talking to the
Switchboard operator. However for many years an operator has not been needed to complete most phone calls. The use of
headsets is becoming more common for placing or receiving a call. Headsets can either come with a cord or be
wireless. A special number can be dialed for
operator assistance, which may be different for local vs. long-distance or international calls.
Details of operation The
landline telephone contains a switchhook (A4) and an alerting device, usually a ringer (A7), that remains connected to the phone line whenever the phone is "
on hook" (i.e. the switch (A4) is open), and other components which are connected when the phone is "
off hook". The off-hook components include a transmitter (microphone, A2), a receiver (speaker, A1), and other circuits for dialing, filtering (A3), and amplification. To place a telephone call, the
calling party picks up the telephone's handset, thereby operating a lever that closes the hook switch (A4). This powers the telephone by connecting the transmission hybrid transformer, as well as the transmitter (microphone) and receiver (speaker) to the line. In this off-hook state, the telephone circuitry has a low resistance of typically less than 300
ohms, which causes the flow of
direct current (DC) in the line (C) from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, and sends
dial tone to indicate its readiness. On a modern
push-button telephone, the caller then presses the number keys to send the telephone number of the destination, the
called party. The keys control a tone generator circuit (not shown) that sends
DTMF tones to the exchange. A
rotary-dial telephone uses
pulse dialing (A5), sending electrical pulses, that the exchange counts to decode each digit of the telephone number. If the called party's line is available, the terminating exchange applies an intermittent alternating current (AC)
ringing signal of 40 to 90 volts to alert the called party of the incoming call. If the called party's line is in use, however, the exchange returns a
busy signal to the calling party. If the called party's line is in use but subscribes to
call waiting service, the exchange sends an intermittent audible tone to the called party to indicate another call. The electromechanical ringer of a telephone (A7) is connected to the line through a
capacitor (A6), which blocks direct current and passes the alternating current of the ringing power. The telephone draws no current when it is on hook, while a DC voltage is continually applied to the line. Exchange circuitry (D2) can send an alternating current down the line to activate the ringer and announce an incoming call. In manual service exchange areas, before dial service was installed, telephones had hand-cranked
magneto generators to generate a ringing voltage back to the exchange or any other telephone on the same line. When a landline telephone is inactive (on hook), the circuitry at the telephone exchange detects the absence of direct current to indicate that the line is not in use. When a party initiates a call to this line, the exchange sends the ringing signal. When the called party picks up the handset, they actuate a double-circuit switchhook (not shown) which may simultaneously disconnect the alerting device and connect the audio circuitry to the line. This, in turn, draws direct current through the line, confirming that the called phone is now active. The exchange circuitry turns off the ring signal, and both telephones are now active and connected through the exchange. The parties may now converse as long as both phones remain off hook. When a party hangs up, placing the handset back on the cradle or hook, direct current ceases in that line, signaling the exchange to disconnect the call. Calls to parties beyond the local exchange are carried over
trunk lines which establish connections between exchanges. In modern telephone networks,
fiber-optic cable and
digital technology are often employed in such connections.
Satellite technology may be used for communication over very long distances. In most landline telephones, the transmitter and receiver (microphone and speaker) are located in the handset, although in a
speakerphone these components may be located in the base or in a separate enclosure. Powered by the line, the microphone (A2) produces a modulated electric current which varies its
frequency and
amplitude in response to the
sound waves arriving at its
diaphragm. The resulting current is transmitted along the telephone line to the local exchange then on to the other phone (via the local exchange or via a larger network), where it passes through the
coil of the receiver (A3). The varying current in the coil produces a corresponding movement of the receiver's diaphragm,
reproducing the original sound waves present at the transmitter. Along with the microphone and speaker, additional circuitry is incorporated to prevent the incoming speaker signal and the outgoing microphone signal from interfering with each other. This is accomplished through a
hybrid coil (A3). The incoming audio signal passes through a resistor (A8) and the primary winding of the coil (A3) which passes it to the speaker (A1). Since the current path A8 – A3 has a far lower impedance than the microphone (A2), virtually all of the incoming signal passes through it and bypasses the microphone. At the same time the DC voltage across the line causes a DC current which is split between the resistor-coil (A8-A3) branch and the microphone-coil (A2-A3) branch. The DC current through the resistor-coil branch has no effect on the incoming audio signal. But the DC current passing through the microphone is turned into AC (in response to voice sounds) which then passes through only the upper branch of the coil's (A3) primary winding, which has far fewer turns than the lower primary winding. This causes a small portion of the microphone output to be fed back to the speaker, while the rest of the AC goes out through the phone line. A
lineman's handset is a telephone designed for testing the telephone network and may be attached directly to aerial lines and other infrastructure components. ==Tones==