Unless the NSA also has speech to text that is decades ahead of everyone else, AND works perfectly well in ~50 languages of interest - because of transcription quality.
Not necessarily - the speech to text just has to be good enough to flag likely use of interesting terms. Those calls could then be fully stored for later analysis by an investigation if it becomes necessary. The vast majority of calls would end up being stored as a text file, but this technique with today's technology would certainly be good enough to flag a reasonably high percentage of calls of interest for audio storage. That phone call from Jimmy to Sally Mae telling her he's going to be late home from work? It doesn't matter if NSA-Siri garbles the translation...