I have the feeling that people who hold loud conversations on mobiles think that they are impressing those around them.
It's difficult to explain - I'm not talking about individuals chatting away quietly in deep conversation, who look totally absorbed in the discussion, but those like AreWeThereYet described who insist that those around them are party to their dialogue.
Like the man on the train once explaining in great detail in a very loud voice the precise stops between Waterloo and Richmond-change-at-Clapham Junction, who had clearly undertaken this journey on a regular basis... "we're nearing the Junction now , do you think Richard is going to bring Bev with him this time, how is she after her trip to Portugal, the platform's crowded tonight on the down-line, what about Harry...". I realised that whoever he was talking to said very little - he or she was basically a prop for his performance. A performance which I felt was meant to impress us with the fact that he.had.a.very.busy.social.life. and knew lots of people, and that one of them needed a blow-by-blow account of his journey to wherever from wherever.