For an upsell to work, you'd first have to believe that people buy iPhones primarily to make phone calls. Then you'd have to believe that these phone-centric individuals are tolerant of the voice latency in GV calls. I don't see it.
I find myself saying "no, go ahead" or talking over the other person far more often with GV than not. It's certainly usable, but I avoid GV if the call quality really matters.