Well the biggest difference is that TWC isn't technically VOIP. It's Digital phone.
I know, I know. Petty distinction, but it does make a difference in this particular instance. Specifically, because the Digital Phone from TWC comes through a dedicated mac (and IP address) to your modem, it takes (virtually) no bandwidth from your ethernet connection (which, in turn, has its own mac and IP addresses). Since it has its own dedicated signal, so long as the modem is getting a strong enough signal, your internet and phone services won't compete for connection.
Of course there are a lot more details than that, and the article I've linked here goes further into detail, but that is about the gist of it.
In the link layer its all SIP, Technology to get the voice is Voice is over IP, physical layer is another story.