My girlfriend is always accusing me of cheating on her when I haven't

Over a year ago I had a female friend. She turned out to be kind of crazy so I decided to stop talking to her. When she would call me I wouldn't answer the phone. This made my girlfriend really suspicious. She would ask me why I wouldn't ever answer that phone number. I told my girlfriend that I don't want to be friends with that other woman, but I don't think she believes me. How can I get my girlfriend to understand?