We can translate spoken words to text strings pretty reliably. AFAIK, we can do this locally without much processing power, but I may be mistaken. From the point that spoken words become strings, chatbots and voicebots are effectively the same.

Those strings are piped through a Switch statement of static responses, or fall out into the Default response. This was the situation in 2013 when I created a speech bot in Powershell, and it sounds like the state of the art here hasn't progressed far.

What's needed is logic to dynamically build the Switch statements, or otherwise better parse human entry and build responses. There has been much work on a few different fronts, but I'm not aware of any which were wildly successful.