Google Voice silently fails for me (like, the important UI parts in help articles are just missing), apparently because I'm not in the US. Oh, I appear to be in a US forum - google led me here, so I'll leave some info for other non-Americans who wander in.
fongo (canada) will email me mp3s, but no transcription yet, and youmail is $5/month for transcripts... it looks like my options are a paid app like that, or a website...
https://spokendata.com/ has a free trial and pretty cheap rates. I could have sworn there was a fully free online transcription service, but I can't find it now.
Oh. spokendata's free trial was... entertaining, but, I'm not sure I would even be able to tell if messages are spam, let alone what they're about. Here's the transcript of a message telling me my new glasses are ready for pickup: "Hi timing disinvestment an optical it's like you know your boss humanity and now be here tonight until five cents. A moral thing here eleven to her eyes and miracle occurred some days. The memory."
Now I understand why most of the search results were about humans, not software, doing the transcription.