Cellphone use is so ubiquitous, and far too many people have great difficulty lifting their heads from the screens of their phones no matter what they are doing. They walk down the street, looking at their phones rather than looking out for others who are walking in their path. When these individuals visit a store, from a wine shop to a book shop, they sometimes continue using their phone even when they go to the register to make their purchase.
That needs to stop!
First, it is rude as the cashier generally needs to engage you in conversation when you make your purchase. It is hard to do so if you are talking to someone on the phone, or texting, or surfing the Internet. Second, it is dehumanizing to the cashier, as your attention is on your phone and you aren't treating the cashier as a human being, but rather as if they were an automaton. Third, mistakes can easily be made as you aren't paying sufficient attention and may not properly understand whatever questions the cashier asks you.
If you have to make a call, then handle the call before you go to the cashier. Once you get in front of the cashier, put your phone away or down. Give your full attention to the cashier. Treat them as a human being. Engage in some actual face to face social interaction. Your life is far greater than the tiny screen of your cell phone.
At the wine shop where I work, this happens on occasion. It is clear from their telephone conversations that it isn't an emergency. And it is more difficult to handle their transaction as asking them relevant questions isn't easy. I know others feel the same way too, and stores are not the only victims. At restaurants, servers can encounter the same problem.
Have some consideration and put your cellphone down.