> 20 Hz
With, what, 400 Hz on top of that? That’s what created the ring sound heard by the caller. When I first heard the sound when farting around on an incoming line, I realized that the audio was connected to the caller while ringing and you could send audio back without picking up the line.
Or add a 5kΩ resistor between tip and ring, the system would think you hadn’t picked up and not start long distance billing...