Comment by modeless

No code actually, I made it all in Twilio Studio which is their visual programming "no-code" tool for phone trees. Clunky to work with but trivial to set up and had enough functionality for this very simple application.

Obviously to make a real app out of it I would have redone it in a real programming language and made some frontends for web/Android/iOS.

Someone else pointed out https://freshbuzzer.com which looks like a real product that does the same thing.