STIR/SHAKEN is a protocol that verifies the authenticity of caller IDs using digital signatures. It consists of two specifications: STIR (Secure Telephone Identity Revisited, IETF RFC 8224/8225) and SHAKEN (Signature-based Handling of Asserted information using toKENs, an ATIS standard). It was created as a standard for technically preventing fraud carried out through caller ID spoofing.
The mechanism works as follows: the originating carrier attaches a digital signature (public-key cryptography) to the caller ID, and the terminating carrier verifies that signature. Verification results are displayed in three levels. A (Full Attestation) means the carrier has fully confirmed the caller is the legitimate user of the number. B (Partial Attestation) means the origin is confirmed but the number's legitimacy is not verified. C (Gateway Attestation) covers cases where verification is difficult, such as international calls. The receiving device displays indicators like "Verified Call," giving users a clue to distinguish spoofed calls.
In the United States it has been rolled out among carriers as a countermeasure against robocalls. Elsewhere, the same mechanism is not running everywhere. Adoption and policy decisions vary widely from country to country, and you cannot assume that a call arriving from abroad has been verified.
Japan has relied not on signature verification but on rules that prohibit sending someone else's number, backed by guidelines agreed among carriers. The Telecommunications Carriers Association (TCA) set up a study group on spoofed caller ID display and maintains two documents: a guideline (second edition) for applying the policy on preventing the transmission of a different telephone number, published by the Ministry of Internal Affairs and Communications on 21 April 2008, and a guideline on countermeasures against spoofed caller ID display (version 3.1), as of August 2026. Fraud using number spoofing apps and spoofed international calls that lead to bank transfer fraud continues, so it remains necessary not to trust the number on the display by itself. STIR/SHAKEN signs call setup over SIP (IP telephony), and Japan's telephone network is structured around NTT with inter-carrier interconnection that differs from the US, so it cannot simply be transplanted. See Caller ID Spoofing - How It Works and How to Spot It for how to respond to suspected spoofing.