I've measured the dial tone several times and earlier the peak was at 440 to 450Hz. At end of the video the 400mV amplitude that I mention is of course peak-to-peak voltage. Note that frequency spectrum may vary for various countries. US dial tone for instance consists of a combination of 350 and 440 Hz signals.
See http://electromotiveforces.blogspot.com/2009/12/telephone-signals.html for more
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