A paid subscription service, such as TrapCall, an app available for both iOS and Android phones, can unmask an incoming private or masked number and any associated caller ID information. Another option is to use call tracing. After receiving a call from a blocked number, dial *57 (or #57 from a mobile phone) and follow your phone company's recorded instructions. After the trace attempt is complete, a recorded message will tell you whether the phone company successfully traced the phone number, name and address of the caller.