It is entirely possible, though not always free or easy, to trace restricted phone calls. The first point of contact should always be the phone carrier because the carrier may have security measures in place or functions available which can stop or track down the source of these calls.
In addition to the service of a carrier's customer service department, there are a few other resources available. In the case of threatening or harassing phone calls, these are criminal offenses which can be prosecuted by law enforcement. If these calls are simply a nuisance, there are a number of private investigators who have the capability of tracing even anonymous phone numbers.