The clerk stood there blankly, not scanning my items. I said, "Er, excuse me?"
"I won't serve you until you hang up; it's rude."
Okay, so they had probably had as bad a day as me, and I can see it's annoying to have to serve people using phones, but I had stopped talking and didn't have it to my ear! What the hell? I was paying full attention to the cashier! *Grumble grumble* I hung up, because I didn't have time to argue it out.
ETA: To clarify, I put the phone in my top pocket; it's a flip phone, so I was obviously still technically in a call, but there was no way I could've continued talking/listening to the person at the other end.