You probably do need to get the police involved to some degree. My nephew had a similar problem a while back and the phone company would only release the phone number to the police.
Here is a pretty good link:
Here is a pretty good link:
According to one entry in that thread it is possible to expose anonymous callerIDs by forwarding to an 800# and then gathering the number from there.
The phone company (AT&T) required a subpoena to release the name/number.