Ah,
But there are already devices on the market that can invisibly add packets to a voice stream over the internet, thereby making it undecipherable, and then strip them out at the far end. So the operators providing voice and data can effectively prevent voice traffic over their data network.
Unless of course you pay a higher data rate - which won't work out too much cheaper than voice rates, and VoIP will still be messier to setup and use (with lesser coverage) than a standard voice call.
I don't think that the big telcos are going to lose out too much with VoIP - in terms of their backbone networks it's all just digital anyway. It becomes a marketing / billing issue instead.
SD