This is one of those widely asked questions in relation to communications technologies that have become part of our daily lives. Mobile phone etiquette hasn’t evolved as fast as phone coverage, so this behaviour remains a regular source of irritation at hotels, restaurants, airports and other public places.
So why do people with normal speaking volumes yell into their cell phones? I came across an interesting explanation, which also suggests that it’s a trait more common among Digital Immigrants.
Here’s an extract: “Household telephones, or landlines, have a microphone in the receiver that amplifies your voice into the ear piece. When you talk into a landline, your voice is captured and replayed through the ear piece, so you hear your own voice loud and clear….With cell phones, your own voice is not amplified into the earpiece, so the only sound you hear is from your mouth. Seem like this wouldn’t be a huge difference, but the volume level of words coming from your mouth through the air and into your ear is a pretty big difference from sounds coming from a phone speaker that’s pressed directly against your ear.”
Of course, the habit goes a long way back to the days when phone lines rarely offered good audio quality. There is the true story of how Sir Winston Churchill had to suffer a Cabinet colleague who was a loud phone talker. During the Second World War, they were sharing crammed war cabins.
One day the Minister was once again talking very loudly on the phone. Churchill asked his secretary to go over and tell Mr Brown not to talk at the top of his voice. The secretary returned and told the PM: ‘Sir, the Minister is talking to Scotland.’
Without batting an eyelid, Churchill replied: ‘Yes, I’m sure he is. But tell him to use the phone!’
Open Minds! (formerly: Moving Images blog) 
Interesting. Do you get the same behavior when you plug headsets ?