There are a couple of voicemail transcription services available, or you can use Twilio. I suggest language like "For quickest response, please spell me your email address."
Google Voice transcribes voicemail. A bit hit-and-miss sometimes, but decent (and, presumably, getting better with more data).
You're being generous... it seems as if their service is worse off now than it was when GrandCentral ran the place.
For me it's usually both accurate enough to get the gist of the message, and inaccurate enough for a good laugh.
I've experienced some hilarious misses when receiving a non-english voicemail and having Google try to make sense of it. It's not so good with fast talkers either.