How to Change Your iPhone's Ringtone and Media Volume Separately
When you press one of the volume buttons on your iPhone, you'll notice that it changes both the media volume and the ringtone ...
Hackaday7y
dual-tone multifrequency
One late night many decades ago, I chanced upon a technical description of the Touch-Tone system ... C64’s three audio channels to generate the dual tones, I bet I can dial the phone!