if you are receiving calls that are withheld, and are a nuisance, you can call your provider straight after each call to have the number logged. They will not be able to give you the number. however what they will say, is go to the police and report the calls, keep a log of times and dates, and once you have enough calls logged (sorry, not sure how many - ask the police) the police can call the network in particular on a specific number only they have and obtain the number/numbers that have been calling you and deal with it in their way.