this is how google voice routes your call over the pstn.... the app calls an 800 number, which then calls whoever you were calling. the alternative would be to be using your data plan to make a voip call, which would also hit a google server somewhere that would dump it into the pstn.

there's a setting in the app under "make and receive calls" that says "use carrier only" or "prefer wi-fi and mobile data" that will switch you between the make a phone call to the 800 number mode and the make a voip call mode. if you're on a data-limited phone plan, and your voice plan is unlimited (aren't all of them, these days?) then "use carrier only" won't eat into your data allowance.