To simply “listen” in on a line, push the button to “monitor”.
To make a call, push the button to “talk”. Some carriers require a pulse to initiate a hook switch pickup. Press the pulse button on the front to get a dial tone.
The top button can be changed from pulse to tone depending on carrier.
The easiest way to trace a live line is to call an automated number announcement circuit. Wikipedia has a list of a few public resources.
https://en.wikipedia.org/wiki/Automatic_number_announcement_circuit
