Why do I keep calling when I know he isn't going to answer??

Here's what to do:
1.Call your friends
2. Talk to someone
3. Go study
4. Eat
5. Drop the phone in the toilet
6. Exercise
7. Talk to family
8. Get engrossed in a movie/youtube/something on the internet.

You're hurting yourself by calling him. Accept the truth that he doesn't want to hear fro you, and move on.