Since this article has hit the web, I noticed the use of muted beeps by telemarketers has nearly disappeared.
All fax machines use modems.
Normally the answering fax machine answers the call with the familiar modem negotiation noise, using the V.whatever protocols. Prior to that, a calling fax will transmit a hailing signal. The muting of beeps used to warn a person they are being recorded matches the profile of the hailing signal. As with any signal, it's possible to mask the warning tones so that a human will not hear them. It is your duty as a citizen to explain to the employees of these scum that their boss has the warning tones turned on but is masking these tones.