[GreenKeys] 1khz reference tone...

[Dennis Boone]

 > Many telephone companies had a 1KHz tone test line.  Here it was
 > always xxx-xxx-9971.  Give your local exchange a try by dialing
 > xxx-9971 and see if the tone is there.

Note that most telco 1kHz test tones are actually slightly off of 1kHz,
because an actual tone of that frequency interacts badly with the
sampling rate of digital phone circuits, possibly resulting in
measurement errors.  The standard telco tone is 1004 Hz.

De