Ahhh.. what has not been discussed are the TWO different types or uses for VoIP.
There is the home user and the business who use TWO different types of the VoIP Communications. Those being described as "Business Class" and "Consumer Class" VoIP which TOTALLY Different.
Business class would not use the "Internet" to handle IP based phone calls as of yet due to the required "Quality of Service" required to maintain a true "Voice Quality" which is in comparrison of the traditional analog circuits. Another benifit is that now the business can use "VoIP" because they are now able to pick up a phone and move thier desk from one office to another with no intervention of the local carriers. Using "General Public" voice over IP would require a LOT of bandwidth to just have a 4 line set up and running. Most business using analog service can save money on both add/move/changes AND a discount in the service costs for the analog lines by switching to a PRI (Primary Rate Interchange), T1 and up, or even a frame relay circuit.
Consumer Class VoIP is basically what has been stated here so far where the "Cell Phone" style of service is acceptable for 90% of the users. Also since it is a one to one converstation what is being established is more of a one to one bridge between the two edge devices using a public infrastructure (Internet).
Oh well.. just figured I would add a twist