How to trace private calls on cell phone?

ok today I recieved a call on my cell from a private number. When I answered it all the girl did was scream then hung up? I dont think its a help thing more like a prank. How can I find out what the number is? Its on att/cingular and im in states

Contact the provider of your cell phone service.

go 2 the police station they are the only 1s that can trace those calls,trust me they have traced me before!!!

can people trace private/restricted numbers by looking at their phone bill and all the call logs? cause I found out my parents can look at who I call. =O but if I prank call someone can they trace it back to my number?