I have it in the St. Louis area. It is not VOIP, but it is an IP based voice system that runs on a proprietary network. Charter's website has a FAQ which answers all of your questions about how it works. Check there for details.
What I can offer is an assessment. It is fine. Good enough to say your decision should be based solely on price/feature comparison. We have it ONLY because of the package deals and the bundling with the Internet. If you use multiple Charter services and you call and ask for the best deal - it generally comes out much cheaper.
FYI - we got it within 2 weeks of it coming out in our area. Took about 2 months to get the 'kinks' worked out, largely because demand was higher than expected. During that time we had instances of people calling and going straight to voicemail or getting 'all circuits are busy' - in all cases Charter was already working on it when we called them and it was resolved shortly. They also invested into expanding capacity quickly. After the 2 months we haven't had an issue. Been about 8-9 months total that we have had it.
Chris