I'm ignoring my friends calls because they don't seem to understand when I told them I am busy.
Why are they trying to call you even if you told them you're busy?
I don't know, I feel like they are being unfair to me.
Friends might not always have a lot of time to talk so if you don't have the ability to it's okay to not always answer their calls. Have you thought about talking to them about not feeling like they are fair towards you?
I wanted to get them back so they will think twice before they call me.
You are doing what they are doing to you. Two negatives doesn't make it right.