Brunette Girl With The Old Phone

Details of operation
The telephone consists of a switchhook (A4) and an alerting device, usually a ringer (A7), that remains connected to the phone line whenever the phone is "on hook" (ie. 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.
A calling party wishing to speak to another party will pick up the telephone's handset, 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 300 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. (Most exchanges are still equipped to handle pulse dialing.) If the called party's line is available, the exchange sends an intermittent ringing signal (about 90 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 phone's ringer (A7) is connected to the line through a capacitor (A6), a device which blocks direct current but passes alternating current. So, the phone draws no current when it is on hook (a DC voltage is continually connected to the line), but exchange circuitry (D2) can send an AC voltage down the line to ring for an incoming call. (When there is no exchange, telephones often have hand-cranked magnetos to make the ringing voltage.) When a landline phone is inactive or "on hook", the circuitry at the telephone exchange detects the absence of direct current and therefore "knows" that the phone is on hook (therefore, only AC current will go through) with only the alerting device electrically connected to the line. 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 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 phones 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.