IP Democracy has a great analysis of the current pricing model that the pure IP VoIP players (i.e. Yahoo, Skype, GizmoProject) are moving forward with.
But I think there is a whole other model that will eventually come into play. That's where they pay you to use the service. While it won't be so transparent, the more added value services you purchase (i.e. voice mail, find me/follow me, unified mailbox messaging, etc.) plus ads that you click through on, merchant club, frequent buyer programs and more that you become a part of, loyalty, affinity and such, all that is data base driven, will result in telephone service being a basic lifestyle amenity.
I keep thinking the mobile guys are a natural for this play also. Imagine having your unused monthly minutes rolled over to your IP calls, and you getting a bonus for calling your friends and family who are on network with you, but the traffic goes IP to IP.
That's the future as I see it, especially with more VoIP via WiFi coming online really soon.