Or some people talk louder when they can't hear the other person on the line. If their line is quiet, doesn't mean you have to over compensate ... Especially in a restaurant 
I think the talking louder thing can happen to just about anybody if the connection is frustrating. It's just that people need to catch themselves doing it, and STFU.
Like I said, I see older people being way more offensive than anyone from a teenager to those in their 40's or 50's. Ringers always topped out, will talk loudly for ten minutes about nothing instead of going outside, or just calling back later.