Have you noticed, how easy it is to be rude when you are talking to a person on phone?
The next time you get one of those annoying cold calls check out your own reaction, and you will know. This heppens because you can get away with rudeness. You don't get to see the other person's reaction and in any case, the caller can't do anything about your being rude; after all, s/he is the one who is intruding, no?
But guess what? It does not take time for this to become a habit. And in no time you appear rude on phone. Even when you do not intend to.
Great communication skills are acquired with constant practice; being firm but polite when talking to total stangers who can do you no harm is a great way to practice your skills.