This idea is too obvious to stake any sort of claim. And once you've decided to do it, there aren't many variations on the theme.

Basically anyone who has programmed with VoIP and has a callbox has thought about or done this. E.g., I did this years ago using a VoIP server that I wrote -- silly prompt sounds and all.

Furthermore, companies like Twilio are seldom surprised by applications of their platform; they likely had to come up with tons of examples for investors when they raised money.