Wow, Alya - that is serious. If I knew the culprit, I'd try catch him in the act. Most criminals get caught through their mistakes. Let them make them. Or tell a responsible friend and get advice. You might know someone who's had that experience. Most people like doing that because they think they have a power over the person if they can illicit some kind of reaction. Don't give them the satisfaction.
Tell your phone supplier, they may be able to get the number blocked. You can be put on a list so that people calling you get put through to a police officer first who screens the callers. Private companies do that too.
Another idea is to make your private information more private on the net, like with Facebook or whatever. Restrict some information. Adjust your privacy settings on social networks you are with. The police can help here a lot too!