Example of how rotary dialing and electro-mechanical relays were used to establish and route a call from the phone number5022245 to the number 5022246:

1. The caller picks up the phone and hears a dial tone. They rotate the rotary dial on the phone to dial the number 5022245.
2. Each number that is dialed generates a series of electrical pulses that are transmitted over the phone line to the local telephone exchange.
3. The electro-mechanical relays in the exchange detect the pulses and use a series of other relays to route the call to the appropriate destination.
4. The first three digits of the number, 502, indicate the local exchange where the phone line is located. The relays in the exchange recognize this and route the call to the appropriate line in that exchange.
5. The next three digits, 246, indicate the specific phone line within that exchange. The relays in the exchange use this information to route the call to the correct line.
6. When the call reaches the destination line, the electro-mechanical relays in the local exchange for that line detect the incoming call and route it to the appropriate phone.
7. The phone at the destination line rings, and the person at that location picks up the phone to answer the call.
8. If the person at the destination line wants to transfer the call to another phone number, they can use the call transfer feature to do so. They would typically dial a special code, followed by the phone number that they want to transfer the call to, and then hang up the phone. The electro-mechanical relays in the exchange would then use a series of other relays to route the call to the new destination.

In-band signaling for Voice calls

is a method of signaling used in telecommunications to control the establishment, maintenance, and teardown of phone calls. It involves using audio signals, such as touch-tones, to control the switching of calls.

example with the frequencies associated with each digit:

1. A user picks up the phone and hears a dial tone.
2. The user dials a phone number using the touch-tone keypad. For example, they dial the number 555-1234.
3. As the user dials each digit, the touch-tone phone generates a specific dual-tone multi-frequency (DTMF) signal that is transmitted over the phone line.

• For the digit 5, the touch-tone phone generates a DTMF signal consisting of a 770 Hz tone and a 1336 Hz tone.
• For the digit 5 again, the touch-tone phone generates a DTMF signal consisting of a 770 Hz tone and a 1336 Hz tone.
• For the digit 5 yet again, the touch-tone phone generates a DTMF signal consisting of a 770 Hz tone and a 1336 Hz tone.
• For the digit 1, the touch-tone phone generates a DTMF signal consisting of a 697 Hz tone and a 1209 Hz tone.
• For the digit 2, the touch-tone phone generates a DTMF signal consisting of a 697 Hz tone and a 1336 Hz tone.
• For the digit 3, the touch-tone phone generates a DTMF signal consisting of a 697 Hz tone and a 1477 Hz tone.
• For the digit 4, the touch-tone phone generates a DTMF signal consisting of a 770 Hz tone and a 1209 Hz tone.

4. The DTMF signal is received by the local exchange, which uses a digital signal processor (DSP) to analyze the signal and determine the appropriate action to take.
5. The DSP uses the DTMF signal to identify the destination phone number and route the call to the appropriate destination.
6. When the call reaches the destination phone, the phone rings and the person at that location can pick up the phone to answer the call.
7. If the user wants to use advanced calling features, such as call waiting or three-way calling, they can do so by pressing specific buttons on the touch-tone keypad. For example, to use call waiting, the user would press the “flash” button on the keypad to put the first call on hold, and then dial the second phone number. The DSP in the local exchange would recognize the call waiting signal and route the call appropriately.