If you use Google Voice, you have the opportunity to help Google transcription research. They're asking for donations, and here's how you can contribute.
When you're reading a transcript of your voicemail (which is already a service of Google Voice, in case you weren't aware), look for text in the lower right corner that says "Transcript Useful?" If you choose to rate the transcript, you'll be asked if you wish to donate the message to Google's research.
If you choose to do so, Google may listen to it and/or transcribe it, but they won't share it publicly.
Image via Google Voice blog