Hey nikolascx, thanks for the detailed explanation.
There are clearly competing needs: (1) the need to quickly inform the user that a new voicemail was received, and (2) the need to provide a transcription.
I have pondered this scenario myself, and I should look at my own code to see how we are handling the same issue. Perhaps, rather than sending a blank transcription, you could put in it's place a message like "voicemail transcription in process. Check your web client for the transcription" or something similar so the user understands the feature is not broken, rather, the transcription service is delayed.
It's a small thing, but good in-app communication can save everyone from spending time researching what looks like a problem, but is actually intentional and by design. I am trying hard to remember that fact and incorporate this approach into everything I do.