It is VoIP, only Comcast controls the entire network so they can apply QOS to the service, which greatly enhances it. They have complete control of the VoIP call from the user to where the call accesses the public phone network.
Other services, such as Vonage and Skype, must use networks completely unknown to them (the internet) and therefore can not control the proper flow of the data (meaning prioritizing voice packets to prevent the delay and breakup of a VoIP call).
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