Designing for chat and voice is a fairly new field so we provide a few tips to keep the dialog engaging and functional:
Being concise and to-the-point is key when you build spoken interfaces, but largely so also for chat. The reason is that users have short attention spans and, specifically for voice, the synthetic voices are not always great at keeping attention. Rather have several shorter turns than a long monologue!
Humans tend to never repeat ourselves word-by-word, and probably by good reason since it drives many of us mad. Few things breaks the illusion of intelligence as much as when the bot repeats the same thing over and over. The counter-measure here is to add variation. It might seem tedious, but it will pay of already during testing since YOU will not be driven mad (at least not as fast as with no variation ;-)).
It is impossible to add all variation to user-input from day one so prepare to iterate fast especially during the first days of an app's life.
Sometimes in conversation, it is acceptable to move on without fully understanding each other. This can definitely be exploited in chat and voice-apps by silently accepting answers if it is not crucial to get an actual measurable answer. For example, it might not be crucial to know if the user had a good day or not if your apps job is to sell flights. So, asking "Sorry, what did you say" if you get unknown input on an introductory "Hi, how are you doing?" question might be unnecessary and a more smooth tactic could be to say "I see", "Interesting" or something else generic and then move on. See error handling for more info and other tactics on handling errors.