Basically, they must be attractive to the callee so the usage is convenient and intuitive. Going forward, the callee should get the impression like they are talking with a human being, not an application, which is not that easy
Recently, I’ve come across an interesting series of posts on The VUI Post weblog suggesting how a dialog should be closed. So far there are 16 pieces of advice, perhaps new ones will appear (will track it).
Here are the links:
If you’re interested in IVR apps, I recommend reading that!