Some callers may be able to initiate a trace on nuisance calls by pressing the "flash" button on their phones, and then pressing "*57." Another option is to install free or paid apps that are sometimes able to trace or unblock private numbers to reveal the caller information.
If the caller is leaving nasty or harassing messages, consider trying to trace the call. Some companies allow users to immediately have a call traced by pressing the "flash" button on their phones and then "*57." However, the information is not released to the user. Instead the company initiates an investigation into the call and may alert the authorities about the nuisance callers.