STIR/SHAKEN and Outbound Spam Labels: What A-Level Attestation Actually Does (and Does Not) Fix

What STIR/SHAKEN Is and Is Not

STIR/SHAKEN is a cryptographic framework for authenticating the origination of telephone calls. When a call is placed, the originating carrier signs a Passport header with a private key, attesting that the calling party is authorized to use the presented calling number. Terminating carriers and analytics providers verify this signature.

It is not a content moderation system. It does not evaluate whether the caller is a legitimate business, whether the call is wanted by the recipient, or whether the call pattern resembles spam. It verifies only that the caller is who they claim to be from an identity standpoint.

The three attestation levels:

• A (Full Attestation): The originating carrier has verified the customer's identity and that the customer is authorized to use the specific calling number. This is the only level that materially affects spam label suppression.
• B (Partial Attestation): The carrier has verified the customer's identity but cannot confirm authorization for the specific number (common when customers use numbers not provisioned by the carrier).
• C (Gateway Attestation): The carrier can only confirm the call entered the network at a specific point, with no verification of the originating party.

Most enterprise SIP trunk providers issue B or C level attestation unless they provision the originating numbers. UnlimCall provisions caller IDs on demand across 33 active markets and signs eligible US and Canadian calls at A-level attestation — because it controls the number inventory and can verify authorization.

What A-Level Attestation Actually Prevents

A-level signed calls on numbers with no complaint history are significantly less likely to receive a "Spam?" or "Scam Likely" label from the three major analytics providers (First Orion, Hiya, TNS). The mechanism:

1. Terminating carrier receives the call with STIR/SHAKEN header
2. Carrier passes the header and calling number to its analytics provider
3. Analytics provider checks: (a) Is the signature valid? (b) Does the calling number have a complaint or pattern flag?
4. If (a) is valid and (b) is clean: call passes without a label
5. If (a) is valid but (b) has a flag: the analytics engine may still apply a label at reduced confidence; A-level reduces the probability but does not eliminate it
6. If (a) is invalid (B, C, or missing): the analytics engine's label probability increases substantially

This is the precise mechanic: A-level attestation suppresses labels on clean numbers and reduces (but does not eliminate) labels on numbers with complaint history.

What A-Level Attestation Cannot Fix

Numbers already flagged by analytics engines. If a number has accumulated complaint reports — through high-volume use, short dwell time on calls, or prior spam association — the analytics engine has its own independent signal. A-level attestation does not clear existing complaint history. To clear a flagged number, you retire it and provision a fresh replacement. On-demand provisioning, as opposed to static number pools, is precisely the solution here: a pool that rotates monthly starts numbers clean.

Consumer opt-out behavior. STIR/SHAKEN has no bearing on whether a prospect decides to answer. It removes a negative signal (spam label) but does not add a positive one. A signed call from a 212 (New York) number to a Seattle prospect still reads as an unknown caller — which is why local presence dialing addresses a different behavioral layer that attestation cannot reach.

International calls. STIR/SHAKEN is a US and Canadian framework. UnlimCall covers 33 markets globally, and attestation framework equivalents (like EU ATIS norms or UK Ofcom authentication initiatives) are in various stages of development. For international outbound outside US/CA, attestation is not the primary connect-rate lever.

Carrier-level blocks (not labels). Some terminating carriers have implemented automated blocking — not just labeling — for calls that fail attestation at scale. If your trunk cannot sign at A-level and you are calling Verizon or AT&T subscribers in volume, a portion of those calls may not reach ringing state at all. This shows up as an ASR drop on specific carrier prefixes, not as a contact rate drop.

The Number Velocity Interaction

Even with A-level attestation, a number dialed at 500 attempts per day will attract analytics-engine attention. The analytics providers score on behavioral signals independent of cryptographic verification — call volume, call duration distribution, complaint ratio, answer rate (counterintuitively, a very high answer rate followed by short calls correlates with robocall patterns). A-level attestation shifts the baseline probability of a label; number velocity management keeps the behavioral score below the labeling threshold.

The local caller ID comparison covers how different providers approach number inventory, rotation, and velocity management — the operational complement to attestation.

Practical Setup for US/CA Compliance

For a US or Canadian outbound operation seeking the lowest possible spam-label rate:

1. Confirm your SIP trunk provider can sign at A-level for all originating numbers — if they cannot, the attestation is B or C regardless of their marketing copy
2. Provision originating numbers through your trunk provider's inventory (not third-party DIDs ported in, which typically drop to B-level)
3. Set a velocity ceiling: no single number should exceed 100 outbound attempts per day
4. Monitor your numbers against major analytics provider databases quarterly (First Orion's Branded Communication, Hiya's Exchange, TransUnion's TruContact all have reputation lookup APIs)
5. Retire and replace any number that accumulates a spam flag within 30 days of flagging