I’m sure this is very painful. It sounds like he has been planning to leave for some time, but you were caught completely by surprise. Your boyfriend is the only person who can answer your question about his behavior. I suspect he doesn’t answer your calls because he doesn’t want you to try to talk him out of leaving.
I think you should calmly ask him to meet with you to help you understand what has come between you. Let him know that as much as you’d like him to change his mind, you won’t put pressure on him. Tell him that you need to hear from his perspective what happened to the love between you. That is the path to forgiveness and to moving on.
Then keep your word. Do your best to simply hear what he has to say. Don’t try to persuade or convince him to come back. Just tell him how you feel about him and ask him to rethink his position. Staying calm will be difficult for you but it is the only way you will get the information you are looking for.
I wish you well.