[CM] telephone noise spectrum

[James Ravan]

http://www.hackfaq.org/telephony/telephone-tone-frequencies.shtml

Above is a link to a page describing all the various tones one might
encounter on a telephone. But in this case, one might also have to deal with
some contamination from the recording process. The lower the quality of the
recording equipment, the higher the likelihood of contamination from that
equipment.

Also consider the possibility that the contaminating signal might be a
square wave. Thus one notch would not eliminate all its contribution.

There might also be more than one contaminating frequency, perhaps from the
same source, perhaps from others.

Of course, if what you're hearing is some broadband noise like an air
conditioner, good luck.

regards,
-jim