A caller ID spoofing app is an application that can change the phone number displayed on the recipient's device to any number of the caller's choosing. Known overseas through services like SpoofCard, SpoofTel, and Dingtone, these apps use VoIP technology to rewrite the caller ID - a technically straightforward process.
Legitimate uses do exist. Companies may display their main office number when employees call from personal mobile phones, virtual phone number services may display acquired numbers, and individuals may hide their personal number for privacy. However, misuse for phone scams and harassment has become a serious social problem. Spoofing police or bank numbers to build trust is a standard tactic in bank transfer fraud, and combining it with voice cloning technology enables even more sophisticated scams.
Under Japanese law, caller ID spoofing for fraudulent purposes may violate the Telecommunications Business Act. However, no law currently prohibits the possession or use of spoofing apps themselves - punishment applies only when they are used in criminal acts such as fraud or threats. In the US, the Truth in Caller ID Act (2009) makes spoofing for fraudulent purposes a federal offense, with fines of up to $10,000 per violation.
As a technical countermeasure, adoption of the STIR/SHAKEN protocol is advancing. This system has originating carriers attach digital signatures to caller IDs, which terminating carriers verify to determine whether the number is legitimate. The US mandated adoption for major carriers in 2021, and Japan's Ministry is also studying implementation. If you receive a call from a spoofed number, do not trust the displayed number - verify the official contact on the organization's website before calling back. See Caller ID Spoofing Risks for details.