I had this problem before, twice: both a few years ago.....
The first time I had no number and didn't know who it was.....My mum reporetd it to my phone company, who then reported it to the police...It took the police over 6 weeks but they came back with the number...turned out it was my (so called) best friend.
I havn't spoken to her since!
The 2nd time, I had the number and he gave me his first name, but I had no idea who he was, he was constantly bombarding me with txts and calls, luckilly in the end tho it just stoppe and i never heard from him again. Odd.
I say report it to the police.