- •STIR/SHAKEN is an FCC-mandated framework that cryptographically verifies a caller has the right to use its number.
- •A-level is full attestation, B-level partial, C-level gateway. Lower levels face more carrier scrutiny and higher termination rates.
- •A-level is not automatic: ported numbers, intermediate carriers, and multi-carrier paths can quietly downgrade attestation.
- •If you cannot see your attestation rate by carrier, you lack full visibility into outbound voice performance.
STIR/SHAKEN is a call authentication framework mandated by the FCC and deployed across North American carriers. Its purpose is to reduce caller ID spoofing by cryptographically verifying that the entity making a call has the right to use the number it is calling from.
For most consumers, it surfaces as a small checkmark or "Caller Verified" indicator on a phone screen. For outbound voice operations, it is a compliance requirement, a performance variable, and increasingly a factor in whether calls reach consumers at all.
What do STIR/SHAKEN terms mean?
STIR stands for Secure Telephone Identity Revisited. SHAKEN stands for Signature-based Handling of Asserted information using toKENs. Together they define the protocol by which carriers sign outbound calls with a digital certificate and pass that certificate through the call signaling so downstream carriers can verify it.
When your call leaves your system, the originating carrier attaches a PASSporT - a digitally signed token - to the SIP INVITE. The token contains the calling number, the called number, a timestamp, and an attestation level. Downstream carriers and analytics engines read that token and use it as one factor in evaluating the call.
The attestation level is where most of the practical impact lives.
What are the three STIR/SHAKEN attestation levels?
| B-level: Partial | C-level: Gateway | A-level: Full | |
|---|---|---|---|
| What carrier verifies | Call origin, but not number authorization | Cannot verify source at all | Caller identity and number ownership |
| Common cause | Number not provisioned through originating carrier | Call received from upstream carrier without attestation | Number directly provisioned, clean routing chain |
| Carrier scrutiny | Elevated | Highest risk profile | Strongest signal |
| Impact on answer rates | Moderate risk to number reputation | Elevated termination rates on some networks | Best foundation for number reputation |
A-level attestation - Full Attestation - means the carrier can verify that the calling party is authorized to use the number, and that the number is a legitimate phone number associated with that customer. This is the strongest signal. It tells the receiving carrier: we know who is calling, we know they own this number, and we have verified both.
B-level attestation - Partial Attestation - means the carrier has verified the call originated from a specific customer, but cannot confirm the customer is authorized to use that specific calling number. This happens commonly when a business is calling from a number that was not directly provisioned through the originating carrier.
C-level attestation - Gateway Attestation - means the call was received from an upstream carrier that did not provide full attestation. The originating carrier cannot verify the source at all. This is the weakest signal and the highest risk profile in the eyes of downstream analytics engines.
The practical difference between A and C attestation is meaningful. Carriers and analytics engines treat low-attestation calls with more scrutiny. On some networks, calls with C-level attestation face elevated termination rates. Number reputation models factor attestation level into their scoring. A number with good call behavior but weak attestation is at higher risk than it should be.
Why is A-level attestation not automatic?
Many organizations assume that if their carrier is STIR/SHAKEN compliant, their calls are getting A-level attestation. This is not always true.
Attestation level depends on how your number inventory is provisioned and the relationship between your numbers and your originating carrier. If your numbers were ported from another carrier and the provisioning records have not been fully updated, the originating carrier may not be able to confirm authorization - resulting in B-level attestation even though you legitimately own the numbers.
Calls that route through intermediate carriers - common in complex enterprise voice architectures or hosted contact center environments - may lose attestation level as they traverse the network. What started as an A-level call from the originating carrier may arrive at the destination as B or C if an intermediate carrier does not properly pass the attestation chain.
Multi-carrier environments are particularly susceptible to this. An outbound operation running traffic through multiple carriers simultaneously may have strong attestation on one path and weak attestation on another without knowing it, because the visibility requires carrier-layer instrumentation.
What does STIR/SHAKEN attestation mean for outbound operations?
For collections, financial services, and other high-volume outbound environments, STIR/SHAKEN attestation is a direct performance variable. Every call is being evaluated by Hiya, First Orion, TNS, and the receiving carrier's own systems. Attestation level is one of the inputs to that evaluation.
An operation running at scale with inconsistent attestation is leaving answer rate on the table. The calls are going out. The infrastructure looks healthy from the inside. But at the carrier layer, a portion of the traffic is being treated with elevated suspicion because the authentication signal is weak.
Monitoring STIR/SHAKEN attestation rates in real time - by carrier, by number pool, by campaign - is part of the same carrier-layer visibility that surfaces 487 rates and early media anomalies. These signals are all generated in the same layer of the network. Operations that can see that layer operate with fundamentally better information than those that cannot.
The Bottom Line
STIR/SHAKEN is not a checkbox compliance item. It is an active performance variable in every outbound call you make. A-level attestation is the standard to aim for, and achieving it consistently requires correct number provisioning, clean carrier routing chains, and real-time monitoring to catch degradation before it affects contact rates.
If you do not know your attestation rate by carrier, you do not have full visibility into your outbound voice performance.
Frequently Asked Questions
What is STIR/SHAKEN?
STIR/SHAKEN is an FCC-mandated call authentication framework deployed across North American carriers that cryptographically verifies a caller has the right to use the number it is calling from. It reduces caller ID spoofing and surfaces as a "Caller Verified" indicator on consumer phone screens.
What is the difference between A-level, B-level, and C-level attestation?
A-level is full attestation: the carrier verifies both caller identity and number ownership. B-level is partial attestation: the carrier confirms the call origin but cannot verify the caller is authorized to use that specific number. C-level is gateway attestation: the carrier received the call from an upstream carrier and cannot verify the source at all.
Why do calls not always receive A-level attestation?
Attestation depends on how numbers are provisioned and the relationship between those numbers and the originating carrier. Ported numbers with incomplete provisioning records, intermediate carriers in complex routing chains, and multi-carrier environments can all cause calls to arrive at the destination with B or C-level attestation even when the originating party legitimately owns the numbers.
How does attestation level affect outbound call performance?
Carriers and analytics engines such as Hiya, First Orion, and TNS use attestation level as one input when evaluating calls. Low-attestation calls face more scrutiny, and on some networks C-level calls face elevated termination rates. Number reputation models also factor attestation level into their scoring, meaning a number with good call behavior but weak attestation carries more risk than it should.
What is a PASSporT in the context of STIR/SHAKEN?
A PASSporT is a digitally signed token attached to the SIP INVITE when a call leaves the originating carrier. It contains the calling number, the called number, a timestamp, and the attestation level. Downstream carriers and analytics engines read this token as one factor in evaluating the call.
How can outbound operations monitor STIR/SHAKEN attestation?
Monitoring attestation requires carrier-layer instrumentation. Operations should track attestation rates in real time by carrier, by number pool, and by campaign. This visibility sits in the same network layer that surfaces 487 rates and early media anomalies, so operations that can see that layer have fundamentally better information about their outbound voice performance.
