People talk at a volume appropriate to their own surroundings, and they forget that the other person on the call isn't in the same environment. You usually have to talk loudly in noisy places - turns out it isn't necessary on the phone, but a lot of people don't realise that.
Also, we are trained to treat "half-conversations" as high priority interrupts. If you hear "Hello!"
"Well hello there Charlie Brown, you blockhead." -- Lucy Van Pelt