Go to your local Police department and file a report. They can go to the carrier and put a tap on your line. Just because 'normal people" cannot see the correct caller id info does not mean the police and carrier can't.
I got a call at 4:00am this morning (May 2 '09) from these people. I don't know who they expect to reach at this hour. They leave a message but it's blank, and no one answers when I call back.
Received a call from this number at 8:35AM on a Sunday morning. I didn't answer and no voicemail was left. My 877 toll-free business line rings to my cell phone, so I don't know if the caller was calling my cell number or my toll-free number.