I too would appreciate a good concise explanation of the benefits etc. I have been thinking of setting up something for callback, but have failed to see the forest for the tree's.
From the tiny bit of research I have done, and by reading
http://www.prepaidgsm.net/forum/call...ading-com.html, I think the main difference between paid and pro seen by people like you and I is callback. I may be wrong, but from what I can make out, in order to setup your own callback server, you need to have at least a Soho account at €3/month. However, that only gives you only one callback channel, and I'm not sure if that is enough. I think there is some confusion about what CLI their system see's depending on whether you are in the country or out.
I'm sure bbob will be able to shed some more light on the matter.