Personally, we don't have our VA take the phone calls, our acquisitions manager does that. Since he is the one who is trained in the aspects of wholesaling, rapport, motivateion, etc, we felt he could be a good gauge of when to set an appointment and when not to. Also, since he is the one going on the appointments, he can only really waste his time. In my opinion, an acquisitions manager/salesman is the first hire.
As far as to your question directly.
I would have a point system for "when they're looking to sell" Yesterday - 5 Today - 4 - Soon 3, etc.
How much work does the property need? Tons - 5 Decent amount - 4, etc
Is it a hot area?
What's their tone like? Sometimes it may seem as though they aren't negotiable by what they say, but their tone says otherwise. Lots of people are easier to deal with in person, so if your numbers aren't all that off it might be a good idea to set an appointment.
Also, something for the point system to consider, maybe, how busy is your schedule at the moment?
Just a few thoughts.