The sound from a phone speaker is annoying, more so, than a typical in person talking. To me the solution lies somewhere in fixing that to make it sound more natural.
Everyone else claiming that some how having “loud” conversation is rude, feels like they’ve fallen into some anti-social hole… we are literally the only animal to have developed complex spoken language… it’s part of our humanity.
The people being quiet in an normally-loud place create no problems. The people being loud in a normally-quiet place are causing problems for others by violating the quiet.
Loud people also tend to be oblivious to this and then get defensive when it’s pointed out. Not always - I’ve known some naturally-loud people who had figured out that being shushed meant they were in the wrong.
There are exceptions to this -- of course nobody expects you to worry about your volume at a concert between sets, at a sporting event, etc. But people who speak very loudly everywhere are annoying to everyone around them.
Most patrons have a conversation at a normal volume where the words are clear to their conversation partner but not to people sitting further away.
Speaking loudly enough to be understood from a significant distance is rude because it prevents other people from having their own conversations, and it forces people who are not having a conversation to listen to you. Speaking at an appropriate volume is not anti-social, it is pro-social: other people can't be social themselves if you're too loud.
The unwritten rules loosen up at night, during events, or at other times when there's a boisterous crowd.
For everything else, the solution is to STFU. People blasting reels or having rambling non-essential phone calls in public transport is detrimental to everybody's stress level and by extension mental and physical health. I'd love to see it banned and the ban actually enforced.
Shout out to the GGT 101 bus driver that made the annoying passenger on some endless legal/business call actually shut up with a polite but firm "Sir, this is a bus, not a call center". Best trip across the Golden Gate Bridge I've ever had.
To the larger point about loud conversations -- any conversations above what is appropriate to the situation, even in person conversations, are annoying. Ever go to a restaurant and you're able to hear the loud table across the room because they're yelling while everyone else is speaking at a normal volume? Highly annoying. "Who ordered the mojito? Monique ordered the mojito!" I'm just trying to enjoy a cocktail and talk with my partner, not listen to your cacophony.
Doubly annoying if you have a speech processing disorder of any kind. I already have a hard time understanding people on one side of my head, I don't need to also be picking up someone's loud voices interrupting my attempts to listen.