Klangfarbe and STIR/SHAKEN: the number, and the agent
STIR/SHAKEN tells you who is allowed to use the calling number. Klangfarbe tells you whose authorized AI agent is talking.
STIR/SHAKEN and Klangfarbe both deal with trust on phone calls, and they answer different questions. One is about the number in the signaling. The other is about the agent in the audio.
What STIR/SHAKEN does
STIR/SHAKEN lets the originating carrier sign the calling number in a SIP Identity header (a PASSporT token). The terminating carrier checks the signature and learns how well the originator knows the caller, expressed as an attestation level. It was built to fight spoofed caller ID.
It stays in the signaling. It says nothing about who or what is speaking once the call connects, and it does not survive into a recording.
What Klangfarbe does
Klangfarbe identifies the AI voice agent speaking in the audio: the organization that authorized it, the agent itself, the authorized purpose and whether the session is valid right now. The binding travels inside the speech, so it can be checked by anyone who has the media stream, including after transcoding and in a recording. Verification continues for the whole call, so a change of speaker is reported as an identity change.
Side by side
| STIR/SHAKEN | Klangfarbe | |
|---|---|---|
| Question answered | Is this caller entitled to this number? | Whose authorized AI agent is speaking? |
| Identity of | The calling number | The organization, agent and purpose |
| Carried in | SIP signaling | The speech, plus optional signaling context |
| Survives into recordings | No | Designed to |
| Checked | Once, at call setup | Continuously during the call |
| Signed by | The originating carrier | The organization and the agent |
| Status | Deployed in several countries | Proof of concept in build |
Better together
Klangfarbe treats signaling identity as useful input, not a competitor. A verified STIR/SHAKEN result or the SIP Call-ID can serve as channel binding, so a Klangfarbe session attestation cannot be moved into another call. A call can then carry both answers: the number is legitimate, and the agent speaking is Acme's authorized claims assistant.
When each one matters
If you are a carrier fighting number spoofing, STIR/SHAKEN is the tool. If you run a contact center that receives calls from AI agents, deploy voice agents yourself, or need to prove later which agent said what on a recording, the question is about the agent, and that is the gap Klangfarbe is designed to fill. See how the protocol works.