lol, the whole beige box thing at the time was a joke, it's just a POTS phone. That's literally just how phones work. It's a DC circuit back to your exchange or to your Private Access Branch eXchange and all terminals are connected in parallel on this circuit share the circuit number; this could be your extension number on a PABX or your subscriber number on your carriers exchange. It was never anything really bespoke like a Blue Box which was a keypad for dialling the full range of DTMF and other carriage tones that didn't exist on a regular handset.
There's a constant 48v DC that drives your terminal; it's caller ID display and associated circuitry, but predominantly the 48v DC powers your microphone, and your 'ring' signal is triggered by a 90v AC voltage placed over the line. Traditionally this would manually drive an armature that would strike an actual bell, now it just triggers an electrical circuit in modern phones. Depending on how much current makes its way back to your location this 90v AC can drive the ringers of a few phones in parallel; too many handsets and none of their ringers will go off.
sauce: actual dog and bone technician; to be honest growing up with all the phreaking stories inspired me but becoming a phone tech really killed the mystique.