I've noticed that people in Poland speak really quietly when on the telephone compared to us Americans, Is there any reason for this?
Yes: yelling on the phone so everyone hears you is rude. Usually people around you do not wish to hear about what you should buy on your way home, your Auntie's health problems, the assignment you've got at work or your problems with your Internet provider. It's your business and yelling about it does not interest the people around you but it can distract and annoy them.
They do that on cell phones, sometimes.
People usually shout on cell phones because their interlocutor can't hear them. I always have problems talking on the phone when I'm on a bus, in the street etc, so the people who talk to me must shout.
In the US it's considered incredibly rude to talk on a cell phone in public or while driving tho people do it all the time.
Well: when at home, you need no mobile phone. When in cinema, theatre, lectures, classes, or at work, you should have your phone turned off. Now, if you can't use it in public - why have a mobile phone at all?