You receive a call from an unknown number and they tell you they've kidnapped your wife. They then give her the phone so you can hear her but she didn't do it through Signal so she's obviously not your wife so you just hang up.
And that's not how you establish a PKI or implement AE (authenticated encryption).
When I send a voice message e.g. I don't verbally sign it, that's actually part of KE (key exchange) protocol.
Same applies to non encrypted, authenticated communication of course.
And in case you are talking about physical face to face communication: I think you'd recognize your family and friends without the help of cryptography.
Edit: if you have any question, feel free to ask! Maybe I misunderstood you? Then I'm sorry
Of course you don't verbally sign voice messages. That wouldn't make any sense at all unless you are a robot or something.
It was just a joke. But if you wanted to authenticate something (like an email, or voice I guess) in a PKI you would sign it using your certificate private key and the CA would tell the recipient if it's valid or not.