With a unit I have been using for a decade or so, This definitely falls into the 'ya learn something new every day" class. Googling REVIEWS let's on what is going on when users comment on how/whether it works

Apparently there were two of these units. An older one (sas?) which sent out a tone and a newer one (adv) which telezapper had to design when telemarketers ignored the tone or their computers stopped responding to it.

The newer one sends out three tones (what I had always regarded wrongly as THE sit tone)

And interestingly for a unit so old, I discovered that there is a 3032 in it which has been in there so long but which seems to have finally died. i am now testing it because as it was "working' for so long in that it sent out the signal, I need to see if anything in the circuit needed power to tell it WHEN to send out the signal.

Anyway, preliminary report is that putting a new battery in it hasn't done much: It still buts into conversations 30-40 seconds after they have started with the annoying tone. I suppose this may mean that the uni has simply died?