Cell Phone Tracing

Hey, i am not sure if this topic has to do with anything on this part of the forums so im sorry if it doesnt. I am just having a little problem here, i have a cingular cell phone and every so often at random hours of the day and at random days of the week i will recieve a call from a person calling ''private''. This person says harassing things like ''look out your window, i see you'' etc etc, bullshit like that. I tried finding ways to trace this persons calls but i was told you cant trace a private incoming call to your cell phone. I was wondering if any of you guys knew any ways to do this? i really want to find out who this person is. It is not only annoying but he/she calls during movies, when im trying to sleep, when i am out with friends etc. Any help would be appreciated, thanks.