It is very simple.
The central office , or telephone company equipment, sends either 48 or 60 volts to the phone on your desk.
That voltage goes trought the two wires (tip and ring),and ends on your phone set on your desk.
When you pick up the phone, that voltage produces a low current to flow trough the phone, signaling the central office that you are off-hook,triyng to make a call.
Youput the handset on your ear, and the central office sends you dial tone, to indiate that is ready to accept your digits.
When the call is dialed, the centrall office inform you of the progreess (or not) by sending audio tones to you. Ringback or busy are the most known ones.
If you hear a busy you know what to do. Hang up and try again.
That is O.K. when it is a central office and a human on the other end.
But when the human is replaced with a device, like a PBX or a dialer, those devices have a hard time to know what the progress of the call is. They do not have a good “ear”
Solution: Battery reversal: When the device snds a call to the central office and the call is done, that means that the other end hangs up (the remote called person hangs up), the cventral office reverses for a couple of seconds the voltage on the line.
That is very easy to detect by a electronic box, and the result is that the originating device hangs up the line, releasing the resources for anther call.
In short, the central office “flips” the tip and ring for a couple of seconds,telling the caller “end of the conversation, hung up the phone”