The Holy Grail for IVR designers is a natural language application that can handle a significant percentage of calls to a service desk. 911 call centers are a kind of mission-critical service desk: you call with your problem and address and hope that help arrives before things get much worse. In order to get natural language to work you'd need to anticipate what callers will say when they call (or at least have categorized a number of previously recorded similar calls).
Unfortunately, there is just no way a VUI designer is going to craft a system that will recognize "alligator in my kitchen" as a legitimate problem call. Even human 911 operators have difficulty with the phrase, as you can hear in this recording.