People can find out who is calling with blocked numbers through the use of third-party mobile applications, such as Trapcall for iPhone and Android devices. Not only can such an app tell users who is calling with a blocked number, but the app can also block such numbers from calling the user again.
TrapCall supports various common carriers in the United States, including T-Mobile, Verizon, Sprint and AT&T. In order for the user to obtain the number of the blocked caller, the user must not answer the call. TrapCall will tell the user the blocked number as soon as the service processes the call. It is important to note that TrapCall cannot look up the numbers of unknown callers, which differ in that they didn't hide their caller ID intentionally but may be using a carrier experiencing call ID issues.