Vonage is Voice Over Internet Protocol (VOIP) and converts the analog signal (voice) into a digital signal and passes it over the existing network via servers.
Comcast says their service is not VOIP because the use their existing fiber network instead of the internet.
A word of warning, all these systems require the use of a modem, which requires electricity to operate. In the event of a power outage (which we never have in Southern Maryland, right) you may be left without service. The Comcast modem has a battery back-up which is supposed to be good for eight hours.