Think about this for a second. Why is this surprising?
I don't know about other people here, but I don't even check my voicemail anymore. Google handles that, and has for years. The voicemail transcription I get through Google Voice is almost always good enough that I can determine who called, what they want, and where to call them back to talk further.
Keep in mind, this is a 'free' service to me, I don't pay anything. Due to the volume of people they do it for, I'm certain they they're trying to meet economies of scale and reducing the overhead. Who do you think funds the storage, equipment, etc. for all of this? Adsense?
And it's no secret that the NSA had early involvement with Google.