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A telephone call, phone call, voice call, or simply a call, is the use of a connection over a telephone network between two parties for audio communication. To start a call, the calling party, the caller, opens a connection for a particular phone number and waits for an answer to the request, often indicated by an audible ringtone. To answer the call, the called party accepts the request to start a conversation. A party is most commonly a single person, but can be a group of people (i.e. conference call) or a machine (i.e. fax). In some contexts, the term A-Number refers to the caller and B-Number refers to the called party.
The telephone call was enabled by multiple inventions in the mid- to late-19th century including the telephone. Initial technology involved point-to-point electrical wire connections between telephone installations, until centralized exchanges evolved where telephone operators established each interconnection manually at a telephone switchboard after asking the calling party for their call destination. After the invention of automatic telephone exchanges in the 1890s, the process became increasingly automated, eventually leading to the widespread adoption of digital exchanges in the second half of the 20th century, including the transition to wireless communication via mobile telephone networks and . With the development of the Internet, the cost of telephone calls was drastically reduced with Voice over Internet Protocol (VoIP).
The first test with promising results was in June 1875 when Bell and his assistant, Thomas Watson, tested the so-called gallows telephone. Its transmitter utilized Bell’s novel concept of vibrating a magnetized part in front of a coil of wire to create an electric current according to the principle of Faraday’s law. This telephone transmitted voice-like sounds (timbre) consisting of many frequencies, and Watson declared that “The gods had arrived.” But distinct words were not heard as had been expected.Bell, Alexander G. 1875-06-30. Letter to his parents. Library of Congress, Alexander Graham Bell Family Papers.
The second test with improved results was in March 1876, and that was the test in which Bell literally shouted, “Mr. Watson – Come here – I want to see you.”Bell, Alexander G. 1876. Experiments made by A. Graham Bell, Vol. I. Library of Congress, Alexander Graham Bell Family Papers. pages 40-41.. Although Watson claimed to have heard those words via telephone, Bell astutely tried to confirm Watson’s claim by changing rooms and having Watson read unfamiliar pages from a book. Bell then wrote “It was certainly the case that articulate sounds proceeded from S the. The effect was loud but indistinct and muffled.” Bell further noted “As it was I could not make out the sense – but an occasional word here and there was quite distinct.” Nevertheless, with reference to those words, in 1916 Bell unveiled a plaque in Boston that reads, “Here Alexander Graham Bell transmitted to Thomas Augustus Watson the first complete and intelligible sentence by telephone March 10, 1876.”
For those tests in March 1876, Bell used a liquid transmitter that appears to be like the one designed by Elisha Gray, Bell’s competitor.Gray, Elisha. 1876. “Talking Telegraph.” U.S. Patent Office Caveat, Filed February 14, 1876. National Museum of American History, Smithsonian Institution, Elisha Gray Papers. It varied the resistance in a battery circuit and was nothing like Bell’s vibrating magnet design. Gray never successfully demonstrated his liquid transmitter, and it was not developed into a practical instrument. Further, Bernard Finn, whose department at the Smithsonian holds Bell’s artifacts, said that Gray’s design “was in fact worthless.”
The third test with successful results was conducted on May 22, 1876, when Edward Wilson was substituting for Watson.Bell, Alexander G. 1876b. Electrical Experiments by AGB, Vol. II. Library of Congress, Alexander Graham Bell Family Papers. page 13. Using Bell’s vibrating-magnet design, Bell said “I could hear very distinctively the sounds that were uttered into A the. Consonants & vowels were equally intelligible. Although my ear was pressed tightly against S’ the – I heard the sentence ‘Mr. Bell are you going to the Centennial’ with the utmost distinctiveness.” This was soon confirmed at the 1876 Centennial Exhibition by demonstrations in front of many witnesses. Further improvements were made to Bell’s vibrating magnet telephone in November 1876, and the improved design was patented and became the world’s first commercial telephone.
If the caller's wired phone is connected directly to the calling party, when the caller takes their telephone off-hook, the calling party's phone will ring. This is called a hot or ringdown. Otherwise, the calling party is usually given a Dial tone to indicate they should begin dialing the desired number. In some (now very rare) cases, if the calling party cannot dial calls directly, they will be connected to an operator who places the call for them.
Calls may be placed through a public network, such as the public switched telephone network (PSTN), provided by a commercial telephone company or a private network, such as a private branch exchange (PBX). In most cases a private network is connected to the public network to provide PBX users with access to the larger networks. Incoming calls to a private network arrive at the PBX in two ways: either directly to a users phone using direct inward dialing or indirectly via a human or electronic receptionist who will answer the call and connect it to the desired user.
Most telephone calls through the PSTN are set up using ISUP signalling messages or one of its variants between telephone exchanges to establish the end to end connection. Calls through PBX networks are set up using QSIG, DPNSS or variants.
A special number can be dialed for operator assistance, which may be different for local vs. long-distance or international calls.
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 , 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 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 Trunking 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 voice coil of the receiver (A3). The varying current in the coil produces a corresponding movement of the receiver's diaphragm, Reproduction 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.
Caller ID provides some protection against unwanted calls, but can still be turned off by the calling party. Even where end-user Caller ID is not available, calls are still logged, both in billing records at the originating telco and via automatic number identification, so the perpetrator's phone number can still be discovered in many cases. However, this does not provide complete protection: harassers can use payphones, in some cases, automatic number identification itself can be spoofed or blocked, and mobile telephone abusers can (at some cost) use "throwaway" phones or SIMs.
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