Wiredog:
VoIP is not a term used "only" to describe service on the public internet. There is no "traditional" VoIP. Voice over the internet protocol is just that, sending voice data packets over the internet. It is what Comcast does, and what Vonage (thought Skype would be a better contrasting example) does.
The only difference is that Comcast only uses the portion of the INTERNET that they control. They do this specifically so that they can apply QoS to it (which you can not do if you do not control the network end to end), and therefore an acceptable quality of service (by programming the network to prioritize voice data packets). Insted of continuing the path through the public internet which they have no control over, they terminate to the public telephone network, for which they have to pay termination fees, and is why the service is more expensive than Skype.
Skype, on the other hand, sends their data packets through the public internet, which can not differentiate between data and voice packets, and therefore delivers sub par performance (though less expensive). If Skype users want to call users on the public phone network, they too pay more for this service, which is why Skype has different costs for internet-internet and internet-public phone number calls.
I continue to argue with you because you continue to claim that Comcast does not use the internet. You fail to realize that their broadband network (the cable infrastructure) is in fact part of the internet.
Incidentally, if you read early Comcast introduction documents to their VoIP service, they made an initial strategic attempt to avoid the use of the term "VoIP" for marketing reasons. They did this because they felt the public would not understand the differences described above, and why Comcasts service was better. They have since adjusted their approach, and are now proudly referring to their service as VoIP. Perhaps this explains your initial confusion with the term?
