There are controls for voice recognition with VB6. You have to download these controls. here is a page that has such a control:
http://www.programmersheaven.com/zone15/cat724/
or for using the microsoft voice recognition control:
http://www.martin2k.co.uk/vb6/tips/q3.php
In order to do what you want to do without using a 3rd party tool (which I'm guessing is the case) you'll have to research API calls. The API may not be able to help you, with visual basic, either, since a lot of API calls require the use of pointers, a data type not support with visual basic. You may have to look at coding the IVR backend in a language like C, and then make the front end visual basic. If you don't think your prof would mind, I would use a third party tool.