Contact your service provider to trace a blocked phone call. You can also trace blocked calls from a smartphone using subscription applications or the *57 function.
When a call is blocked, contact the telephone service provider and give them the date and time of the call. Provide them with as much information as possible, including whether the call was a telemarketer or someone sending harassing messages. If a telemarketing agency contacts someone registered with the national Do Not Call list, the company can be fined.
Applications available for smartphones can provide a blocked caller's identity. When activated by pressing the power button twice, apps such as TrapCall trace calls from blocked or unknown numbers. TrapCall provides the offending phone number as well as the name and address of the person or business calling. TrapCall has a subscription cost of around $5 a month, as of 2015.
Any blocked call accompanied by a threatening message should be dealt with by the police. After the call occurs, hang up the phone. When a dial tone comes back on, press "*57"; a recording indicating that call tracing is activated then plays. Contact the local authorities and report the incident in detail. When further calls occur, the information is forwarded to the authorities.