Telephone call
Telephone calls started in the late 19th century, initially relying on analog systems.
As technology advanced, the majority of telephone calls moved from traditional landlines to cellular networks and mobile phones.
Bell demonstrated his ability to "talk with electricity" by transmitting a call to his assistant, Thomas Watson.
Incoming calls to a private network arrive at the PBX in two ways: either directly to a users phone using a DDI number or indirectly via a receptionist who will answer the call first and then manually put the caller through to the desired user on the PBX.
[4] Other services, such as toll-free dial-around, enable callers to initiate a telephone call through a third party without exchanging phone numbers.
The off-hook components include a transmitter (microphone, A2), a receiver (speaker, A1), and other circuits for dialing, filtering (A3), and amplification.
This powers the telephone by connecting the transmission hybrid transformer, as well as the transmitter (microphone) and receiver (speaker) to the line.
The exchange detects this current, attaches a digit receiver circuit to the line, and sends dial tone to indicate its readiness.
Exchange circuitry (D2) can send an alternating current down the line to activate the ringer and announce an incoming call.
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.
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.
In modern telephone networks, fiber-optic cable and digital technology are often employed in such connections.
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 varying current in the coil produces a corresponding movement of the receiver's diaphragm, reproducing the original sound waves present at the transmitter.
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.
Preceding, during, and after a traditional telephone call is placed, certain tones signify the progress and status of the telephone call: Cell phones generally do not use dial tones, because the technology used to transmit the dialed number is different from a landline.
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.
