Denon HEOS also has a fully documented API intended for people to use to build cool stuff! Sonos has no interest in this, so everything has had to be reverse engineered - and could conceivably break at any time. Iâ(TM)ve built custom integrations with HEOS speakers that I never would have been able to do with Sonos. https://support.denon.com/app/...

This is true, at least in the US. That three-tone beep is called the "SIT tone". It is a standard cue that machinery can detect to find out when the number they just dialed is out of service.

Click here to hear a WAV file of the tone.

You can go here for the real TeleZapper website, which is a device that detects when you answer your phone, and plays the tone basically as you're saying "hello".

Asterisk has this sound built-in, so you can trigger it in your dialplan wherever you need.

Wikipedia has great information on the Special Information Tone, and other call progress tones that you probably just take for granted.