I don't know about the implementation, but I would imagine it's like the "Now Playing" feature, where the phone has local hashes or ML models for current popula

I don't know about the implementation, but I would imagine it's like the "Now Playing" feature, where the phone has local hashes or ML models for current popular music, or common phrases you might hear on the phone. It could also just be that my phone knows when it's calling voicemail, and maybe there is out of band info telling the phone app that it wants a PIN.