Etiquette is a strange and wonderful thing. There's something magical about the idea that random rules of behaviour can neatly separate polite people from the riffraff, the well-bred from the plebs.
Bread rolls at the table must always be torn, don't you know; never cut. And when people say "How do you do?" one must never reply "Great, thanks" - not exactly sure why; it must be a rhetorical question I guess. Never say toilet; it's a "lavatory" or "loo" - even "bog" is acceptable. Who'd have thought? And never hold a knife like a pen; you should hold a knife like a knife.
But while all these traditional niceties have long been set in stone, the accepted etiquettes surrounding the use of smartphones and mobile technology in general are emerging as we speak. Some people interpret the fact that we're all still figuring out what's acceptable and what's not as licence to abandon all principles of decent behaviour. This stance is a cop-out for there are some basic points that we really should all adhere to.
Do not take or make a telephone call at the checkout.
I've seen people talking on the phone while being served by a shop assistant or checkout operator. This behaviour is beyond rude. If your business was really that crucial I'm sure you'd also be important enough to have an assistant or a housekeeper to do the grocery shopping for you. So if you're not engaged with matters of great urgency such as saving the planet from imminent destruction, I have just ten words of advice while at the supermarket checkout or shop counter: Put the phone down and step away from the phone.