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Glossary · Regulation

STIR/SHAKEN

In one sentence

STIR/SHAKEN is a set of standards that lets the originating carrier digitally sign a call to vouch that the caller is entitled to the number shown, so terminating carriers can detect spoofed caller ID.

Also known as : STIR/SHAKEN · STIR SHAKEN · caller ID authentication · call authentication · CLI authentication · CLI spoofing · caller ID spoofing · number spoofing

What is STIR/SHAKEN?

Caller ID spoofing means showing the person you call a number that isn’t yours: a bank’s, a council’s or a real company’s. Fraudsters rely on it because people trust a familiar number. STIR/SHAKEN is the technical answer adopted first in North America: STIR (Secure Telephone Identity Revisited) is the IETF standard that defines how to sign a call, and SHAKEN is the industry framework that sets out how carriers use those signatures between their networks.

The idea is simple. The carrier that originates a call checks its customer’s right to the caller ID being presented, then attaches a digitally signed token. The carrier that delivers the call verifies the signature before showing the number.

How it works

  1. Your PBX sends a call presenting a number.
  2. Your carrier checks the number against your account and adds a signed token, called a PASSporT, to the SIP signalling, with an attestation level: A (full), B (partial) or C (gateway).
  3. Any transit carriers pass the token along unchanged.
  4. The terminating carrier verifies the signature using the originating carrier’s certificate. Calls that fail, or arrive with low attestation, may be labelled, sent to voicemail or blocked depending on local rules.

Example

A debt advice charity in Birmingham runs outbound calls through a hosted PBX and also contacts clients in the US. Some of its US calls showed as “Spam likely” because they arrived via a gateway carrier with C-level attestation. By routing them through a provider that holds the charity’s numbers and can sign them directly, the calls carry stronger attestation and are less likely to be flagged. In the UK, the charity’s main focus remains presenting only numbers it owns, in line with Ofcom’s CLI rules.

What it means for your business

  • Present only numbers allocated to your business, ideally held by the carrier that sends your outbound calls over your SIP trunk.
  • If you use several carriers or an outsourced call centre, check who signs your calls and with which numbers.
  • In Ireland, ComReg has required operators to block certain calls from abroad that spoof Irish numbers; in the UK, providers must already block calls with invalid CLI.
  • Marketing calls must also respect consent rules and the TPS.

Call authentication at Universal Telecom

With our SIP trunk or virtual PBX, your outbound calls present numbers allocated to you. If calls fail or show the wrong number, contact us and we will check the configuration.

Frequently asked questions

Where is STIR/SHAKEN mandatory?

The US required most voice providers to implement it from June 2021 and Canada followed later that year. France introduced its own number authentication scheme based on the same standards in 2024. Ofcom has said it intends to bring in CLI authentication in the UK once networks have moved to IP.

What do the attestation levels A, B and C mean?

A (full) means the carrier knows the customer and that they are entitled to the number. B (partial) means it knows the customer but not their right to that number. C (gateway) means the carrier only knows where it received the call from, typically from abroad.

Does STIR/SHAKEN stop all nuisance calls?

No. It proves the number shown hasn't been spoofed; it doesn't judge whether the call is wanted. Nuisance calls from genuine numbers are handled by consent rules and preference services such as the TPS.

Related terms

  • Caller ID (CLI)

    Caller ID, or CLI (calling line identification), is the caller's number passed through the network and shown to the person called. A business may only present a valid number that it is entitled to use.

  • SIP trunk

    A SIP trunk is a voice-over-IP connection between a business phone system and a carrier. It replaces ISDN lines and carries calls and numbers over your internet connection.

  • SIP

    SIP (Session Initiation Protocol) is the standard signalling protocol that sets up, changes and ends VoIP calls. It handles ringing, answering and hanging up; the voice itself travels separately.

  • TPS (Telephone Preference Service)

    The Telephone Preference Service (TPS) is the UK's free opt-out register for unsolicited live sales and marketing calls. Under PECR, businesses must not make such calls to registered numbers unless the person has told that caller they are happy to be called.

  • Geographic number

    A geographic number is a landline number whose area code points to a place, such as 020 for London or 01 for Dublin. It signals a local presence and is charged as a standard national call.

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