I think there's two sides to this. Obviously there are lots of very good people working in call centres, but there's also some extremely poor ones and a lot of practices that go to infuriate. Frankly, I think customer services in this country is abysmal.
I think everyone has had the experience where they're on hold for 20 minutes and being told every 30 seconds how important their call is. That's guaranteed to raise the blood pressure, particularly if the person calling is at the end of their wits because their salary hasn't gone in, or something.
Then you have people who can only re-iterate stock answers and go to pieces when they have to try and think outside of the box.
Then you have people who have no authority to actually do anything. Normally, you talk to a supervisor and they'll do whatever you wanted when you called - so why wasn't it done in the first place?
What about when you have a problem, they take details of the call, say they'll get back to you then don't? Then you phone up again and they've got no record of the previous conversation so just reiterate the same thing again...
Although there is never any excuse for threatening the staff, call centres can be extremely frustrating places.