When you’re bang-smack in the middle of talking to someone and they answer the phone without telling you first.
Their phone is in their pocket on vibrate, so you don’t hear it ring. You’re in the middle of a perfectly good conversation when this person suddenly, out of nowhere, whips his phone out and answers it “HELLO”.
That’s very rude. He doesn’t need permission to answer his phone, but it would be courteous to say “oh one minute please” before he does..