I'm seeing lots of people recommending the use of SI tones ["SIT tones" is tautological].
I'm in Canada, first, so that may have some strange influence. Second, I'm on our national do not call registry - noting that one of the first things our government did was sell the list to spammers and robocallers, increasing the volume of nuisance calls for 100% of the people who supposedly "opted out".
About 10 years ago, I set my answering machine up with its default outgoing message beginning with the "Intercept" (number changed or disconnected) SIT. Having collected call statistics since before then, I can say with a high degree degree of certainty that **NO automated callers have EVER respected the SI tones**. Ever. Not a single one.
YMMV, of course.