We can make it more difficult for those who broadcast their every move from their seat in the bus or their place in the supermarket queue.
How about taking out your own mobile and having a imaginary one-sided conversation? If it is in the supermarket it could be about how you will be late meeting up because someone in front of you is talking on the phone and holding up the queue. On the bus it could be on any subject whatsoever.
If you were feeling brave you could relay what you have just heard and say that it is fascinating what you can learn about a stranger. If you were really brave you could plan a bank heist, tell your imaginary friend every detail of a night of passion, recite the whole of some long boring poem, anything at all that would bring home to the offender that their every word can be heard by a dozen people.
I would not recommend this approach late at night when you have to walk deserted streets to get home, but the truth is that we CAN do something about it, if we feel strongly enough about it. If it were condemned whenever it happened, it would become more rare, like spitting in the street.
(Not sure I am brave enough to do it, unless REALLY annoyed.)