I consider them an intrusion into my right to my own environment. I use public transit a lot and it amazes me how many people feel compelled to call someone to chat while they are enroute. I can see the need if they are expecting to be met on the other end of the trip but a casual - and loud - conversation is ridiculous. Especially since a good number of them are serial offenders- quite often a person hangs up then immediately dials someone else.

One of these times I'm going to try Ron Olesko's suggestion. That ought to do it. :)