It's not "a virtual number that works with any carrier." I can assure you that the number is (1) natively homed or (2) ported - to a particular physical, facilities-based carrier (generally a CLEC) at any time. In a lot of places, Google Voice is using bandwidth.com's new CLEC license holdings (after they stopped being a pure Level3 reseller), and probably the usual national-level multibillion dollar folks that are interconnected to everyone, everywhere and have a lot of direct end-office trunking: Global Crossing, XO, Level3, etc.
