[kijin]

> (i.e. what if my ringback tone is a busy signal?)

My father used to take the handset off the hook during important family meetings. Anyone who called during that time got a busy signal. This was long before ringback tones were a thing.

I've also heard of a similar technique being used to fight telemarketers a few years ago. Many telemarketers run a program that automatically detects nonexistent numbers and removes them from the list. So this guy made an app to imitate the phone company's 404 error msg (or whatever the audio equivalent is) whenever he received a call from a known telemarketer. I don't know whether he did it with a ringback tone or just played an mp3 after picking up, but anyway, he no longer received any calls from telemarketers after a while.

[krupan]

You make me feel old. Yes, it was in the late '90s and early 2000's that you could buy the Telezapper. It was a little box that connected between the wall jack and your phone and played the little three-tone "number disconnected" sound whenever you picked up your phone. The telemarketer's computer would then (so they said) register your number as disconnected and they would never call you back. If a human called they would hear the tone too and sometimes hang up right away thinking they had mis-dialed. Small price to pay to get rid of telemarketer calls, though :-)

[kijin]

Found it! http://www.k3pgp.org/telezap.htm

Another trick was to play the fax tone. But as the link above says, you risked getting spam faxes if you played the fax tone.