1. I post a question.
2. Someone elects themselves as a candidate to answer that question.
3. I receive notification of the candidate and approve them
4. A call happens
Do you have any idea around what sort of time lag would be inherent in each step? Do you expect steps 1-4 to happen within a half-hour window and thus be mostly instantaneous as far as phone interactions go or could it go longer?
If you expect a quick response then I don't know if your rates are high enough to keep people actively watching the question board instead of checking in a few times a day - if not then I think the usability of the product may suffer since scheduling the call could become problematic. If I have a meeting in an hour should I bother trying to set up this call or do I need to wait until I have several free consecutive hours?