My cable company provides Voip phone service. The advantage for me: it's cheaper.
But:
It gives me exactly the same functionality as a regular phone line.
Why
A) Doesn't it give me the option of calling/receiving calls when I'm on the internet somewhere else.
B) doesn't it allow me to call to skype and sip phones for free? It only allows me to call regular phone numbers. I know skype is not an open protocol.

Such options would give real cost savings, because calling costs could go down.