Maybe the person on the other end keeps saying "I can't hear you". I know if I shouted that would be why. Actually I'd sooner tell them I'll phone them back.
Or do you mean those people who are having a smooth conversation that isn't being interrupted by bad reception, and just want the entire street/bus to hear? They're just attention seekers, they get off on you looking at them. Well, that's what I guess because I would think it would be embarrassing to have strangers hear your phone conversation.