Technically A LOT of backbone and almost ANY LD carrier utilizes VoIP nowadays. It jsut tends to be transpartent to any and all users.
When you say "involved" do you mean deveolping and refinning it or utilizing it on their netowrks or selling minutes on their network or ???
I used to sell millions of mins per month and dare I say knew the state of VoIP "pretty well" This was a year ago or so and Im sure things have changed dramatically at this point.
Certainly for long haul LD, PSTN traffic is dying hard. However, rest assured that wireless/cell phone traffic is alrady killing VoIP and traditional PSTN traffic at an alarming rate.