What I've seen alot of people want is more than 2 channels on a flat rate did. People are scared of per minute no matter what you tell them.
I wonder if you could do something like a flat rate for 2 channels and for every channel you add you get an aditional x number of minutes added to the soft cap.

I'm thinking about something with a fixed number of channels that would be a bit cheaper than the vPRI offerings offered buy some voip providers.