In short, I don't have a lot of faith in this being plausible yet. The major reason is the phone lines. Phone call quality is not solid enough to ensure a accuracy rate high enough to roll this out as a production service. There are others, like legal considerations, but if I had to pick one that would be it.
There's a reason why this rose to number one HN, people would clamor for it. To think that Apple and Google haven't been thinking the same thing is short sited.