The principle that telephone lines can simultaneously transmit power and signals without affecting each other.

There is direct current on the telephone line, which is -48V when it is not off-hook, and it will drop to 8~ 12V after it is off-hook (because the telephone line is powered by constant current). On this basis, an alternating audio signal (during the call) is superimposed. The signaling is like this: when someone calls you, the subscriber circuit of the exchange gives the called party (your phone) a ringing current (25 Hz 75V) superimposed on -48V DC, and the phone rings. When you pick up the handle, the switch stops ringing when it detects off-hook (DC load increases) and switches to the voice channel.

In another case, the caller, when you pick up the handle, the switch knows (DC load increases) and sends you an audio (called dial tone) waiting for you to dial (dual tone multi-frequency or intermittent pulse). Once receiving the signal, the switch will cut off the dial tone and start receiving numbers, ready to switch your call to your designated phone. After receiving the number signal analysis, they will call the other party.

There is DC voltage between telephone lines when there is no call, and DC+ AC voltage when there is a call.