Radium wrote:
You
input a 1 KHz tone into the microphone, telephone lines will carry a 1
KHz AC current to the destination. The louder the sound into the
microphone, the stronger the amperage in the telephone lines.

On a side note, its actually voltage modulation towards the subscribe and
current modulation back to the central office. The earpiece is a high
impedance (2,000 ohm) device that responds to voltage variations. The
carbon microphone element 220 to 200 ohms modulates the talk battery current.