Exactly. And I’ve never understood folks who can’t just do nothing for a few minutes… while they wait, while they pick up their bags, whatever. I don’t ever want to be in that much a need of stimulation, and I say this as someone who has their phone attached at the hip. My game / spouse / Instagram / you-name-it can wait until I get out of line. To me, that’s same sort of RIGHT NOW attitude is what leads to texting-while-driving deaths. 
So, in this situation, you would have me hang up, and then just stand there and stare at the cashier? “Hold on. I’ll call you back. I’ve gotta do ‘nothing’ for a few minutes.” That just seems unnecessary and a little odd.
I’m not, because I am busy putting my items into the cart (No bags, please!) but I figure the time I save them by not making them bag makes up for the extra seconds I take to get back to the card slider machine.
Huh, am I the only one whose supermarket still has baggers?
Oh no, there are baggers, but they can’t be trusted. I divide my stuff up into bag-volume sections which go together but even if the cashier keeps them in those sections (which they don’t), the bagger doesn’t. There’s no way I could juggle a phone and the kind of OCD fit I have around my grocery sections. 
There are places that do have baggers, but it’s not the standard around here. (NYC/Long Island) Though I am talking about large supermarkets, not places like Trader Joe’s & Whole Foods, which are set-up in a completely different way.
Agreed. Or disagreed. Whatever supports my point 
Email or a telephone conversation require your full attention. Speaking with the kids (or someone else who is there in person with you) is a bit different. It’s like the difference between a driver talking to a passenger in the vehicle vs. talking on a cell phone. Studies have shown that the cell phone converstation requires more of a person’s attention and focus than in-person conversations (perhaps because the speaker is imagining the other person’s physical reactions).
When in a cab I will follow the cabby’s lead. If they want to talk, I’ll talk. If it appears after iniitial pleasentries that they prefer to remain quiet I can do that, too. Same for barbers, doctors and other such interactions.
My sole point is that the person in front of you deserves one’s full attention whenever possible. That is what I consider “common courtesy”. You can never go wrong by making everyone you deal with feel important.
So you believe that any other person’s desire to have a conversation automatically overrides any desire you may have to not have a conversation? That conversation trumps all? I think that goes beyond common courtesy to being a pushover.
What if you’re on an 8 hour flight and the person next to you wants to discuss Obama’s birthplace the entire trip?
It’s about treating a living breathing person, doing you a service, like a machine. Or, like they were invisible.
Not cool. Not ever.
Whether you can see it or not, everyone else sees a self important douche. And says as much, when you depart. But I’m sure that doesn’t concern some people.
They truly don’t care if they offend others, make the service person feel bad, or appear publicly a douche. It’s themselves alone they care about, ultimately. There have always been, and will always be, such people.
Just pity them and be thankful you’re not one of them!
But save your breath. They don’t want to hear it, and ultimately don’t care. Period. The value of the ‘unnecessary social nicety’ of treating a service person, as an actual human being, is beyond there ability to comprehend, I believe.
I don’t start or continue a phone conversation while doing something else like interacting with a cashier or clerk. However, I have received incoming calls while in line or in a store and they were important enough that I didn’t want to ignore them. I try to complete the call quickly, or tell the caller where I am; either please wait a minute, or I’ll call them back. It serves all of us poorly to try and juggle several things at once.
::sigh:: You just keep pushing out further and further from the topic proposed by the OP. We started in a store interacting (or not) with a clerk performing a transaction that I initiated, then we moved to plumbers and doctors, now you’ve got me on a plane next to a talkative stranger. Next up, do I speak to a serial killer just before he rams the knife in my chest? I’ve given my opinion, you are welcome to yours (which, according to the poll, is in the vast minority). As elbows says, however, we will talk about you when you leave.
I know I keep pushing, but that’s just because I’m curious what your threshhold is. However, you have my full and abject apology for doing so. I think we can agree that at some point, courtesy does not require you to continue a conversation with a person you’ve been thrown into contact with. I was honestly surprised you said you think that courtesy requires you to have a conversation with a cabbie if they desire one - in that case, practically no resident of New York city is courteous.
And as I said from the beginning, you should be polite to the cashier when you get to the front of the line, and you should be ready to conclude the transaction when they’ve completed their job. If you’ve only got a few items, then no, there’s no time to pull out your phone. But if it’s really going to take several minutes for them to ring everything up and bag it, I do not find it discourteous to do something other than wait for them to start a conversation.