IVR “for dummies” & features comparison tools?

I am evaluating IVR solutions/vendors for an education company that currently uses an IVR solution for assessment, but is rapidly outgrowing it (currently at 35K users, anticipated 100K next year.) I’m just learning about IVR systems/vendors and need a crash course and tools for evaluating features & services. If anyone has guidance to give, please let me know. Thanks!