Phone numbers that are blocked using the star key and the number 67 can always be traced by the phone company and authorities. The call blocking feature is part of a caller ID system, and it is only blocked to the user answering the phone. From an end-user standpoint, there is also software that claims to be able to reveal blocked numbers.
A person that uses the call blocking feature uses it so that others cannot see the phone number that they are calling from. However, if they have been harassing or threatening others, they are not immune to being revealed or being prosecuted because they are using the call block feature.
In addition to call blocking, most phone companies also offer call trace. To activate this service, users must dial star and the number 57 immediately after the call. Depending on the company, this may be subject to a small fee, and it may have to be activated. Typically, the phone number is not revealed to the customer and is kept for professional use only. Call Trap is another feature that shows blocked numbers to the proper authorities. In addition, paid software, such as TrapCall, claims to be able to reveal any blocked number to the end user.