When you do telephone calls, I figure that it's important to
understand that some will cooperate and be courteous, but
there will always be some who don't want to be bothered.

I'm sure that you are polite.
And the work itself is valuable to your employer and those
who you and your employer represent.

Actually, I am thankful that there are call centres - because
these provide employment opportunities to lots of people;
and working in a call centre is also a great opportunity to
practice "customer service" in the way you would like it to
be provided for yourself.

Don't let any person on the other end of the phone make you
feel lousy. Just know that you are doing a good job, and the
nice will notice this. Develop that thick skin when dealing with
the unpleasant ones.
All the best to you.