How do you make a friend stop calling you?

To start with, you could just ask them. They may not realize they are being a pest. This is especially true if you are younger, and your friend is just getting phone rights.

If they are displaying an obsessive behavior, you can't. A person with this condition sees ANY response as positive, so you have to ignore them. They don't think normally so a normal response is not possible. Get the book A Gift of Fear by Gavin de Becker and read chapter 8 starting on page 119. His advice works.