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In later decades their analog cellular system evolved into digital networks with greater capability and lower cost. Convergence has given most modern cell phones capabilities far beyond simple voice conversation.


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Most are smartphones , integrating all mobile communication and many computing needs. A traditional landline telephone system, also known as plain old telephone service POTS , commonly carries both control and audio signals on the same twisted pair C in diagram of insulated wires, the telephone line. The control and signaling equipment consists of three components, the ringer, the hookswitch, and a dial. The ringer, or beeper, light or other device A7 , alerts the user to incoming calls. The hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call.

A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the s dials used almost exclusively the rotary technology, which was replaced by dual-tone multi-frequency signaling DTMF with pushbutton telephones A4. A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the incoming and outgoing speech signals on a single pair of wires.

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A twisted pair line rejects electromagnetic interference EMI and crosstalk better than a single wire or an untwisted pair. The strong outgoing speech signal from the microphone transmitter does not overpower the weaker incoming speaker receiver signal with sidetone because a hybrid coil A3 and other components compensate the imbalance. The junction box B arrests lightning B2 and adjusts the line's resistance B1 to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths A8. The line voltages are negative compared to earth, to reduce galvanic corrosion.

Negative voltage attracts positive metal ions toward the wires.

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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. The off-hook components include a transmitter microphone, A2 , a receiver speaker, A1 , and other circuits for dialing, filtering A3 , and amplification. A calling party wishing to speak to another party will pick up the telephone's handset, thereby operating a lever which closes the switchhook A4 , which powers the telephone by connecting the transmitter microphone , receiver speaker , and related audio components to the line.

The off-hook circuitry has a low resistance less than ohms which causes a direct current DC , which comes down the line C from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, and sends a dial tone to indicate readiness. On a modern push-button telephone , the caller then presses the number keys to send the telephone number of the called party. The keys control a tone generator circuit not shown that makes DTMF tones that the exchange receives. A rotary-dial telephone uses pulse dialing , sending electrical pulses, that the exchange can count to get the telephone number as of many exchanges were still equipped to handle pulse dialing.

If the called party's line is available, the exchange sends an intermittent ringing signal about 75 volts alternating current AC in North America and UK and 60 volts in Germany to alert the called party to an incoming call. If the called party's line is in use, the exchange returns a busy signal to the calling party. However, if the called party's line is in use but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call. The ringer of a telephone A7 is connected to the line through a capacitor A6 , which blocks direct current but passes the alternating current of the ringing signal.

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 AC current down the line to activate the ringer and announce an incoming call.

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When there is no automatic exchange, telephones have hand-cranked magnetos 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 the called party picks up the handset, they actuate a double-circuit switchhook not shown which may simultaneously disconnects the alerting device and connects 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.

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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 current 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 current 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.

Before the development of the electric telephone, the term "telephone" was applied to other inventions, and not all early researchers of the electrical device called it "telephone". Perhaps the earliest use of the word for a communications system was the telephon of Gottfried Huth in Huth proposed an alternative to the optical telegraph of Claude Chappe in which the operators in the signalling towers would shout to each other by means of what he called "speaking tubes", but would now be called giant megaphones.

This instrument used four air horns to communicate with vessels in foggy weather. Johann Philipp Reis used the term in reference to his invention, commonly known as the Reis telephone , in c. His device appears to be the first device based on conversion of sound into electrical impulses. The term telephone was adopted into the vocabulary of many languages. Credit for the invention of the electric telephone is frequently disputed.


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As with other influential inventions such as radio , television , the light bulb , and the computer , several inventors pioneered experimental work on voice transmission over a wire and improved on each other's ideas. New controversies over the issue still arise from time to time. That first patent by Bell was the master patent of the telephone, from which other patents for electric telephone devices and features flowed. In the United Kingdom the blower is used as a slang term for a telephone. The term came from navy slang for a speaking tube.

Early telephones were technically diverse. Some used a water microphone , some had a metal diaphragm that induced current in an electromagnet wound around a permanent magnet, and some were dynamic — their diaphragm vibrated a coil of wire in the field of a permanent magnet or the coil vibrated the diaphragm. The sound-powered dynamic variants survived in small numbers through the 20th century in military and maritime applications, where its ability to create its own electrical power was crucial.

The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument. Early telephones were locally powered, using either a dynamic transmitter or by the powering of a transmitter with a local battery. One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery.

During the 20th century, telephones powered from the telephone exchange over the same wires that carried the voice signals became common. Early telephones used a single wire for the subscriber's line, with ground return used to complete the circuit as used in telegraphs. The earliest dynamic telephones also had only one port opening for sound, with the user alternately listening and speaking or rather, shouting into the same hole.

Sometimes the instruments were operated in pairs at each end, making conversation more convenient but also more expensive. At first, the benefits of a telephone exchange were not exploited. Instead telephones were leased in pairs to a subscriber , who had to arrange for a telegraph contractor to construct a line between them, for example between a home and a shop.

Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones.


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Western Union , already using telegraph exchanges, quickly extended the principle to its telephones in New York City and San Francisco , and Bell was not slow in appreciating the potential. Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator , by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell in a ringer box , first operated over a second wire, and later over the same wire, but with a condenser capacitor in series with the bell coil to allow the AC ringer signal through while still blocking DC keeping the phone " on hook ".

Telephones connected to the earliest Strowger switch automatic exchanges had seven wires, one for the knife switch , one for each telegraph key , one for the bell, one for the push-button and two for speaking.

Large wall telephones in the early 20th century usually incorporated the bell, and separate bell boxes for desk phones dwindled away in the middle of the century. Rural and other telephones that were not on a common battery exchange had a magneto hand-cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.

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