Phones were originally something you held up to the side of your face so that the earpiece was next to your ear. But now I see many people holding their smartphone horizontally, in front of their mouth. Clutching it weirdly so that it doesn’t fall off, because it’s not resting on their cheek. And frowning because they can’t hear what the other person is saying.
What am I missing? Are they all on video calls? What’s the point of a video call where you’re showing the ceiling of the supermarket?
About five years ago we had a meeting. You weren’t invited. The topic was “How can we maximize irritation for @Heracles in entirely mundane ways?”
This was one of the top action items that resulted from the meeting. Do you want to see if you can guess what the rest of them were?
Well, as the old saying goes, “Different strokes for different folks.” If I get a call while I’m curled up on the couch watching TV, I like to balance the phone on my left shoulder. My hair covers it and holds it in place. I turn down or mute the sound so I can listen, talk, and surf all at once. 
Smartphones are have not been designed to be ear cradled, and as I’m sure you know its a bit awkward and you often feel the flat middle against one’s cheek, instead of the ear contact. I suspect that, along with the common switch to use of speakerphone lends itself to the star trek communicator hold you mention.
Come on, we all have some mundane things that irritate us even though others may not even notice them or care if they did. LOL
I do it because the volume of the ear speaker on my iPhone has gone to hell.
They are on speaker phone.
They were raised by wolves.
Follow them around enough and you will see them going through checkout, continuing their speakerphone conversation, and only pausing for a moment to pay for their purchases, only after waiting for a natural pause in the conversation to do so.
I’ve been seeing this for a few years now, across all ages, and I do admit, it bugs me a little bit in public because I just feel awkward hearing someone’s conversation like that. Like I was in the elevator the other day, and the person on the other end was talking about whopping some child’s ass for misbehaving or something, and I just never asked to be part of this conversation. So that annoys me. I don’t want to hear other people’s business. I’m not naturally nosy. I know the same can happen just standing next to two people having a conversation, but at least most people try to moderate what they’re talking about when they know strangers can overhear, and also, that speakerphone is just harder to ignore.
If people are choosing to have public conversations by holding their phones this way and using the speaker, what’s to stop you from joining in? 
No one can hear me if I don’t do this. The phone was designed for absolutely everything except, you know, phone calls. It’s impossible to hold the phone against your face in the normal way you used to hold a phone, and expect to both hear and be heard. I switch positions back and forth – holding it out speaking directly into the mic (the tiny pinhole in the bottom edge) and then moving the top speaker area back near my ear to listen. It’s kind of a drag but I swear I’m not doing it to annoy anyone.
“$10 million dollars”
“ATM machine”
“Citibank, a bank” (in The Economist)
Ilium and Olympos by Dan Simmons, a complex story where multiple characters know what’s going on but somehow never reveal anything beyond what’s needed for the next 5 minutes of action.
My phone is a Star Trek communicator.
To me, that comes across as posing for a camera Kartrashian-style. Sure isn’t practical. Of course, I tend to find speaker-phones in public a bit rude anyway, both to the involuntary audience and to the person on the other end who may not be aware that their private conversation is being blasted to an audience of strangers.
I was visiting my mother in Florida on the weekend and found myself in a factory outlet mall where my niece had been over the Christmas holidays and she wanted a particular blanket she had seen. I had to FaceTime her and show her the blankets so she could pick one.
I won’t deny that I was a bit mortified.
Then you are woefully out of date! 
I suspect this is a lot of the same dynamic as people holding guns sideways when they shoot, or why they fall down when they get shot in the arm. They see people doing it, they assume that’s how it’s done, can’t imagine it being anywhere else.
On behave of all members of the subspecies canis lupus, I protest this denigrating stereotype. 
A couple of weeks ago I was at work and went to the boys’ room…there was someone in a stall having a long loud conversation on their phone. It was a large bathroom with multiple stalls and lots of flushing going on. There is no way the other listener couldn’t tell where the caller was.
I imagined saying in a firm tone “Dude, are you actually talking to someone on the phone while seated on the commode?” or something less genteel. But it was work and who knows who that guy was.
Great guesses! You left out a few, though. Some may be yet to come…
I’m sure smart phones just work better that way. I still hold mine like old landlines but that’s just because I’m old (note the two spaces between my sentences…)
ETA: I just noticed the stupid Internet removes my extra space…grumble, grumble…)