This post is WAY overdue because this is seriously one of the most annoying things you can ever do.
Do not ever go up to a register to check out while you are on your phone. Put your phone down or in your pocket at the VERY least. Don't continue your stupid conversation while you're on the phone. If you have to, just stand somewhere in the store and finish your conversation before you come up to the register.
Or do what most rational people do and say, "Hey sorry, I'm about to check out at Old Navy, can I call you right back?"
I think cashiers hate this because it's kind of the epitome of rude customers who don't treat service workers like real people. It's like you think you're going through a U-Scan at the grocery, only, um, you're not.
In a more practical sense, though, the fact that you're on the phone hinders my ability to do my job. I have things to ask you and tell you. When you're on the phone, we can't have the conversation we need to have.
Following up on this, it's almost just as rude to check out in the middle of a conversation. You're basically forcing me to interrupt you.