Introduction
STIR/SHAKEN is the system that decides whether your outbound calls look trustworthy or suspicious. Get flagged with a low attestation level, and your calls show up as "Spam Likely" before anyone even answers. This guide covers what the attestation levels actually mean, why full attestation still isn't a guarantee, and the real steps that improve your score. By the end, you'll know exactly why your calls are getting blocked and what to do about it.
- STIR/SHAKEN grades every outbound call with an attestation level of A, B, or C.
- Full attestation helps, but it's only one signal carriers use to decide if a call is spam.
- Calling patterns and complaint history can still get an A-rated number flagged.
- Checking your attestation level is the first step to fixing blocked calls, not the last one.
What Is STIR/SHAKEN?
STIR/SHAKEN is a caller ID authentication framework built to stop spoofed calls. STIR handles the technical signing of a call's origin. SHAKEN sets the rules for how carriers apply that signature across their networks. Together, they let a receiving carrier check if a caller ID is real before your phone even rings.
STIR vs. SHAKEN: Two Parts of One System
STIR stands for Secure Telephone Identity Revisited. It's the technical standard for signing a call cryptographically. SHAKEN stands for Signature-based Handling of Asserted information using toKENs. It's the framework that tells carriers how to apply STIR consistently.
Why This System Exists
Caller ID used to be easy to fake. Scammers spoofed local numbers to trick people into answering. Billions of robocalls hit US phones every year before carriers stepped in. This framework exists specifically to close that gap.
Before this system, any provider could stamp any number on an outbound call. A scammer in another country could show up as your neighbor's exact phone number. Cryptographic signing removes that trick entirely, since a spoofed number simply can't carry a valid signature from a carrier that never actually issued it.
What Are STIR/SHAKEN Attestation Levels?
Every outbound call gets tagged with an attestation level before it leaves your provider's network. That level tells the receiving carrier how confident your provider is in your identity.
Level A: Full Attestation
Your provider knows who you are. They've confirmed you're authorized to use that specific caller ID. Most phones show this as a checkmark or a "Verified" tag right on the call screen. This is the level every business should be aiming for.
Level B: Partial Attestation
Your provider knows your account, but not the specific number. This often happens with verified caller ID setups that aren't fully tied to your account. A call center using a shared trunk without per-number verification usually lands here. It's not blocked outright, but it carries more risk than Level A.
Level C: Gateway Attestation
Your provider has no relationship with you at all. The call passed through as a gateway connection, often from an international carrier handing off to a US network. This is the level most likely to get flagged or blocked outright. Very few legitimate business calls should ever carry this label, and seeing it consistently on your outbound traffic is a clear signal to switch providers.
How Caller ID Authentication Actually Works
This process happens in the background on every call you make, in four quick steps.
- 1Your PBX or SIP trunking provider sends the call request.
- 2Your carrier checks its own records for that number and account.
- 3The carrier signs the call with an attestation level and a digital token.
- 4The receiving carrier verifies that token before the phone even rings.
None of this requires you to do anything manually, on a call-by-call basis. It happens automatically, every time, as long as your provider actually supports it correctly. A provider that skips this step silently leaves every one of your calls unsigned, which almost always reads as higher risk on the receiving end.
What Shows Up on the Receiving Phone
The attestation level itself is invisible to most callers. What they actually see is a carrier's interpretation of it. A verified checkmark, a business name, or a plain "Spam Likely" tag are all downstream decisions built on top of your attestation level. Two carriers can treat the exact same Level A call differently, since each one runs its own scoring on top of the signature. That's why a call can look clean on one network and get flagged on another.
This is exactly why testing on more than one carrier matters. A call that shows up clean on your own phone might still get flagged on a different network entirely. Larger businesses often test across two or three major carriers before trusting a "problem solved" verdict from just one test call.
Why Your Calls Get Blocked, Even With Full Attestation
Most guides stop at "get Level A and you're fine." That's not the whole story.
Attestation Is Only One Signal
A full attestation still isn't a guarantee. Carriers combine it with calling patterns, complaint history, and how old your number is — a layered approach consistent with the FCC's call-authentication rules, which set the floor rather than the ceiling. A brand-new number making thousands of short calls a day still gets flagged, even at Level A.
What Actually Triggers a Spam Label
Volume spikes are the biggest trigger. Calling the same numbers repeatedly is another. Short call durations, high abandon rates, and a fresh number with no history all add risk on top of your attestation level.
A Real Example
Picture a 30-agent sales team with fully verified Level A numbers. They dial 2,000 calls a day from the same ten lines. Average call length sits under 15 seconds, since most calls go straight to voicemail. Within two weeks, half those numbers show up as "Spam Likely" anyway. Full attestation never stood a chance against that calling pattern.
Which Industries Feel This Most
Outbound-heavy industries get hit hardest. Insurance, real estate, and collections all dial high volumes from a small pool of numbers. Healthcare reminder lines run into the same problem during appointment season. Any team calling the same list repeatedly, fast, is a prime candidate for a spam label, no matter how clean their attestation looks on paper. Slowing the pace and spreading calls across more verified numbers usually fixes it faster than any single technical setting ever could.
How to Check and Improve Your Attestation Level
Start by calling your own cell phone from your business line. Most carriers now show the attestation result directly on the screen. If you see "Spam Likely" or no verified badge, ask your provider for your current level in writing. Do this test from every number your business actively dials from, not just your main line.
Improving it usually means just one thing: give your provider more proof that you truly own the number.
- Verify ownership directly through your provider's portal.
- Avoid third-party number leasing that breaks the ownership chain.
- Keep account details current whenever a number changes hands internally.
- Ask your provider for a written attestation report, not just a verbal promise.
Ringflow's platform ties every outbound number to a verified account by default, so attestation stays clean without extra paperwork.
Rotating your outbound numbers also matters here. Pair verified ownership with local presence dialing done correctly, and you avoid the number burnout that drags down even fully attested lines. A rotated pool of verified numbers beats one heavily-used number every single time. Review that pool monthly, not just once at setup, since a healthy number today can still burn out three months from now.
STIR/SHAKEN Compliance: What the Law Requires
The FCC requires STIR/SHAKEN implementation from all major voice providers in the United States. This isn't optional infrastructure. It's federal policy aimed at cutting down illegal robocalls network-wide. Providers that skip it risk real penalties, not just angry customers.
Smaller providers had extended deadlines, but most of the industry is compliant today. That doesn't mean every provider implements it well, though. Some sign calls correctly but rarely reach Level A for their customers. Ringflow builds this compliance directly into its contact center platform, so outbound calls carry proper attestation without your team managing it manually.
International calls add another wrinkle. A call originating overseas and terminating in the US often can't carry the same signature a domestic call gets. Gateway providers bridging that international traffic are exactly where Level C attestation shows up most often, and it's rarely something a single business can fix on its own.
Conclusion
STIR/SHAKEN decides how trustworthy your caller ID looks before anyone answers. Attestation levels A, B, and C set the baseline for that trust. Full attestation helps a lot, but it won't fix bad calling patterns on its own. Checking your level and cleaning up your calling habits both matter.
Blocked calls cost you real conversations every single day. Start by checking your current attestation level this week. Ringflow's platform keeps that level clean and pairs it with smarter outbound routing. Your next call deserves to actually reach someone.
Ready when you are
A Blocked Call Never Gets the Chance to Be a Good Conversation
Ringflow keeps your STIR/SHAKEN attestation clean and your outbound calls connecting.
Frequently Asked Questions
It's a caller ID authentication framework that verifies calls are coming from who they claim to be. It stops spoofed numbers from reaching your phone with a fake identity.
It applies to voice calls carried across US telecom networks, covering both mobile and landline carriers. International calls routed through US carriers are affected too.
Large carriers had to comply starting in mid-2021. Smaller providers followed on extended deadlines through 2023.
Call your own cell phone from your business line and look at the screen. Most carriers display a verified badge or a spam warning based on the result.
No, attestation is one signal among many that carriers use. Calling patterns, complaint history, and number age all factor in separately.
No, attestation is tied to identity verification, not timing. Calling hours and time zones are a separate compliance issue entirely.
Most providers include proper attestation as part of standard service, not a paid add-on. Costs only appear if you need a dedicated number verification service on top.
They assume Level A alone guarantees delivery and stop watching their calling patterns. A verified number with bad calling habits still gets flagged as spam.
