How STIR/SHAKEN Attestation Affects Outbound Answer Rates

The Mechanism: How Attestation Becomes a Display Decision

When an outbound call arrives at T-Mobile, AT&T, Verizon, or any other terminating carrier in the US or Canada, the carrier's call analytics platform scores the call in real time. That score pulls from multiple data sources: the STIR/SHAKEN PASSporT embedded in the SIP Identity header, consumer complaint databases, call behavior history for that number, and the originating carrier's reputation score.

The attestation level — A, B, or C — is an input to that scoring model, not the only input. An A-level call from a number that has generated 500 consumer complaints in the past 30 days will still get labeled. A B-level call from a number with a clean history and normal handle times will usually deliver clean. But attestation level shifts the prior. It sets the baseline trust the analytics engine starts from before it adds behavioral signals.

The practical outcome is that A-level attestation raises the threshold at which a call gets labeled or screened. Your B-level calls need a cleaner behavioral profile to achieve the same delivery result.

What the Data Actually Shows

No carrier publishes labeling rates broken down by attestation level, so there is no single authoritative number. What practitioners consistently report across large outbound operations:

• Answer rates on fresh numbers with A-level attestation typically run 15–25% higher than on numbers with B-level attestation, all else equal
• B-level numbers that accumulate behavioral risk factors (high daily volume, short duration calls, consumer complaints) degrade to "Spam Risk" display status 2–4x faster than A-level numbers under equivalent behavioral load
• C-level traffic to T-Mobile and certain AT&T segments experiences meaningful post-dial delay, which measurably reduces connect rates independent of labeling

These are practitioner-reported estimates from large outbound operations, not peer-reviewed studies. Your results will vary based on campaign type, geographic concentration, and calling window.

The Number Rotation Problem

Many outbound teams respond to labeling by rotating caller IDs. They burn through a pool of numbers, retiring flagged DIDs and presenting fresh ones. This approach has three problems.

First, it is reactive. By the time a number appears labeled to your agents, it has already been labeled for an unknown number of calls. You are burning revenue before you detect the problem.

Second, if your SIP trunking provider issues numbers from a shared pool — numbers that have been used by other customers before you — you may be inheriting behavioral history you did not create. A number that looks fresh to your system may have C-level attestation history or complaint flags from its previous user.

Third, rotation does not fix the underlying attestation level. If your provider can only issue B-level attestation because they do not directly control the numbers in their pool, rotating to a new number does not change that. You get a clean behavioral slate on a number that still starts at a trust disadvantage.

UnlimCall's on-demand caller ID model issues numbers that are owned and managed within our network across 33 markets. When you request a caller ID for a new campaign in the US or Canada, you are getting a number with a clean history and the structural prerequisite for A-level attestation — not a recycled pool number.

Attestation's Role in Mobile vs. Landline Delivery

STIR/SHAKEN's impact is concentrated on mobile delivery. Landline termination, business phone systems, and VoIP endpoints that do not participate in STIR/SHAKEN validation are unaffected — the attestation signal never reaches a display system that acts on it.

For B2B outbound calling where you are dialing office numbers and PBXs, attestation level matters less in the short term. The main carrier spam analytics systems are primarily mobile-focused. If your campaign targets mobile numbers — consumer sales, collections, appointment reminders, insurance outreach — attestation is a first-order variable. If you are primarily dialing business main lines, it is a secondary concern.

UnlimCall covers 33 markets spanning both consumer and B2B dialing footprints. Understanding which segment of your list hits mobile vs. business landlines informs how aggressively you need to pursue A-level attestation on a per-campaign basis.

Measuring Attestation's Impact in Your Own Operation

You cannot optimize what you cannot measure. To isolate attestation's contribution to answer rate in your own campaigns:

Split your outbound traffic across two sets of caller IDs — one group where you have confirmed A-level attestation and one group at B-level. Run equivalent lists, equivalent calling windows, equivalent handle times. Measure connect rate (not just answer rate — some analytics trigger post-dial screening that inflates ring time before the call fails). After 10,000 dials per segment, you will have a clean read on the delta in your specific environment.

Most dialer platforms and SIP trunking providers do not expose attestation level in their standard reporting. You may need to pull SIP traces or ask your provider directly what attestation level your traffic is signed at.