Why do mobile phone users shout? Why do cellphone users shout into it as if it were a tin can connected to another by a string? There's a scientific reason.
I used to be a telephone designer. In fact one of my claims to fame is that I redesigned the original glow-in-the-dark dial "Trimphone" to create the push-button version. But I digress!
In the very early days of phone communications, people used to shout. It really was "tin can" technology in those days and you had to shout. Or they thought so.
Later, when technology improved, people continued to shout and manufacturers wondered why.
It turns out that it's all dependent on what you hear.
If you are in a room with no furnishings, your voice sounds very loud and you tend to speak quietly. If you are in a room with lots of furnishings, you tend to speak more loudly as your voice seems to be "absorbed by the walls".
Likewise, when you speak to a telephone, if you can't hear your own voice coming out of the earpiece, your brain tells you that you are whispering, so you end up shouting!
Modern telephones are designed with a "sidetone transformer" which feeds back your voice from the microphone to the earpiece. For technical reasons, this doesn't seem to be possible with mobile phones. The result is that people tend to shout.
But it's not necessary and you can educate yourself not to do it. Next time you use your mobile phone, try speaking very quietly into it. Say "can you hear me OK?" Invariably the other party will say "yes, fine". Mobile phones have very sensitive microphones that will pick up a whisper at ten paces. They also have a circuit that will reduce the volume if you shout. But, I repeat, THERE'S NO NEED TO SHOUT!
Speak quietly, and give your neighbours a break.