So, I guess this is like going to Freenode#RubyOnRails, but for people who don't know what IRC is?
You should absolutely, totally, utterly figure out a way to charge for this service.
You should absolutely, totally, utterly figure out a way to charge for this service.
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.
Props to these guys for doing it for free! I hope it works out for you.
One thought: you currently suggest that the experts who volunteer get paid for their support. As an alternative model, you might consider providing credits for priority support. In other words: experts answer the easier questions so support staff don't have to, and then those experts get priority access to ask trickier questions and/or bug reports. Developers really want to interact with those experts anyway, without opening themselves up to handholding-level support questions, so everybody wins.
As an aside, you may also want to make it more clear why your service works better than existing croudsourced support, such as community forums. I think you can tell a pretty compelling story there: less effort to maintain the community resources (since you do the work for them) and phone support rather than forums/lists/etc. The idea of talking to an expert on the phone often seems far more appealing than sending off a mail or a forum post, at least for certain types of questions.