How do you react to private calls?

This private number has been calling me for many months. At first I thought it was someone who wanted to sell me something, so I didn’t pick it up. Now it has been several months and I haven’t been answering the calls. Sometimes they call me several times a day. A friend of mine once answered the call, but then they hung up! I can’t pick up the phone at this point, as it has been several months. But man, I really would like to know what the hell is going on.

Has this ever happened to you? Could it be that it is just a seller?