The people who are doing this are mental and they get some sick satisfaction knowing that they are making you mad!
I have an idea just hang up the phone and go about your day.
Find something to be happy about.
These people have a very difficult life already and it is getting worse this is so sad but they choose this lifestyle.
They think that doing this it is making them feel better probably picked on at school or did not get the respect that they think they deserve?
I got to tell you though I can not think of to many things that would make you more of a COWARD than to hide behind a phone, spoof your number, and act stupid.