You can have the carrier identify malicious external callers. You can save the caller's station number during the call or for 30 seconds after the call ends. However, it is essential that you do not replace your handset during this time. 

Press the key. The LED lights up.  

Select and confirm. 

Enter the code. 

After you have finished tracing the call, the data is stored on the carrier's system. Now contact System Support.