Callback

There is a number of a person I don't know who has been calling me several times for a week. I sent him a message asking why he is calling and who he is, but he did not respond. I blocked him, but he still tries to call multiple times in a row without stopping. The call does not come through because he is blocked, but the app shows me that he tried to call. This made me suspicious; it's the first time a number insists on calling like this!!