Sounds like you have a secret admirer who is making your life miserable.
You don't have a lot of options. You can't block the number because you don't know the number. You can't use call return, either.
Does your phone service offer call tracing? If so, they will be able to trace the number and after a few calls, they will send the caller a letter asking him (or her) to stop and if the calls continue, phone priviledges will be suspended.
Or do what you don't want to do and get another number.......