This isn't just pixel phones anymore, and it hasn't been for a while. You can also tell that it's not a person sometimes (specifically extended car warranty for me) if you have a cheap phone like mine, because you can hear the message on the other end start as soon as the line opens, before the phone shuts off the speaker and starts playing it's own message; once the assistant finishes talking, you can see the transcript start in the middle because the other end doesn't recognize Google assistant like it (probably) would an answering machine.