Sure you can use Vonage for phone instead of AT&T (or whatever), but you can only do that because you have a pipe to the internet. VoIP services are means of getting phone over a different pipe, but you still need a pipe.
You also seem to miss the point of my comment: governments should have policies like mandatory local-loop unbundling (also called line-sharing) which produce competition where it makes sense (providing backbone) instead of where it makes little sense (providing the "last-mile" connection).
Finally, a duopoly is better than a monopoly, but not by much. It still gives the players far too much control of the market.
You're missing the point, services like voip could end up priced arbitrarily in the interest of the carrier. What if non-AT&T voip services were priced out of the market? This is not beneficial to the consumer. It's easy to claim wireless or some new technology will always provide innovation and options, now, when the market is new and less consolidated. 10 years from now, there's no reason to think a single company won't own cell networks as well as cable networks, or any other medium for that matter. The status quo sucks! I don't know about your area, but around here ISPs rarely, if ever invest in infrastructure, and upgrades are unheard of. My parents in a rural area have been waiting for years for any land based broadband options, 3g doesn't even work there!
ATT used to say that the phone network was "a natural monopoly". Today we'd laugh at that. Really, this example is so glaring that I'm surprised you'd offer this argument. And in fact you acknowledge that the local loop is separate from the actual network. But the difference in the US is that this is a free market -- I can choose to use Embarq (the traditional copper-wire circuit-switched carrier), or I can (and do) use Vonage, or scads of other IP-based carriers, or I could do like many people do and just use cellular. We the consumers are getting to choose just how this unbundling happens.
If, for some reason, you think that there's something sacred about telephone, while data is different (or vice versa), there are other counterexamples as well.
In New York, while electrical transmission remains a monopoly, generation has many choices. It works just like the phones: where the infrastructure demands a single source, that's what we get, but that doesn't mean that the people that provide the wires can be the only one involved in the business. (I pay more for electricity that's generated in part by windmills)
And most obviously, it seems to me that most people have access to two broadband carriers, albeit their capacity varies widely (but that's what markets are all about, right?). Virtually everyone I know has a choice of a DSL provider or a Cable provider.