It's difficult to enter the world of telecom IVR, the barrier of entry is very high, and to do anything at a large scale requires a large up-front investment.

Twilio makes telecom IVR as simple as writing a web application, and they're very affordable with no up-front costs.

I've written custom emergency notification systems, call-in contests, intelligent call forwarding systems (you could replicate Google Voice). Any phone app you can think of can be constructed.