If this looks useful to you, you might also want to see Anveo and their visual call flow editor. The UI isn't quite as polished, but it's been around quite a while and has a lot of features and integration points. It's not open source...runs on Anveo's servers.

Couple of screenshots:

http://www.anveo.com/features/images/variable_sample1.png

http://www.anveo.com/features/images/callflowsample.png

Edit: No association with Anveo other than being a customer. There are very few services that provide a drag/drop IVR or call flow builder with very low end pricing, so I thought it worth mentioning.