How would you make an automated telephone hotline?

I've been kicking around some ideas for an app that would work by having the user dial in over the phone. I don't really intend to go through with it but I keep thinking about how such a thing would work in practice.

Like how does the I/O work there where a computer is able to receive/send audio from a phone line?

How would such a computer answer the phone in a way that doesn't ever result in a busy signal if many people call this hotline?