The major distinction here is voice is much more personal and one on one than IRC. IRC can be intimidating for a novice, and let's be quite frank, many times people's IRC persona does not reflect who they are in real life. As someone who's been contributing to major networks such as EFnet for many years now - I've learned IRC can be a great tool for learning, but it can also be downright aggressive and humiliating, particularly to the majority of users who this service would be targeting.

A voice on the other hand is more personal, synchronous and on-demand. I think there's value there and that it's possible to charge for that additional value.