You dial a US number, it rings, and the recipient sees Spam Likely above your company name. Nothing is broken — a carrier looked at your call, could not satisfy itself that the caller ID was really yours, and said so on screen.
What the system is for
Caller ID was designed when only telephone companies could originate calls, so it was never verified — the number you present is simply the number you claim. That assumption stopped holding the moment anyone could originate a call over SIP.
STIR/SHAKEN closes it by signing. The carrier that puts your call on the network attaches a cryptographic token saying who handed it over and how confident they are that the caller ID belongs to them. Every carrier downstream can check that token.
The three attestation levels
- A — full. The originating carrier knows you and knows the number is yours. This is the one to aim for.
- B — partial. They know you, but cannot confirm you have the right to that particular number. Common when you present a number bought elsewhere.
- C — gateway. They know only where the call arrived from. Usually international traffic entering the US, and the most likely to be labelled or blocked.
If your calls arrive as C and you are presenting US caller ID, that is the thing to fix first, and it is fixable.
A-level is not a promise the label will not appearAttestation tells a carrier the caller ID is genuine. It says nothing about whether the call is wanted. Analytics engines sit on top of it and watch behaviour as well, so a properly signed call can still be flagged.
What gets you labelled even with an A
The labelling itself is done by analytics companies whose data the carriers license. They watch patterns, and these are the ones that trip the threshold:
- Very short average duration. Lots of calls ending in a few seconds reads as people rejecting you.
- Low answer rate. High volume with few connections looks like dialling a list.
- One number, huge volume. Spread outbound across a sensible range instead of hammering a single CLI.
- A number with a history. Recycled US numbers carry the previous holder’s reputation. Check a number before you build a campaign on it.
- Complaints. Recipients can report a number in a couple of taps, and it counts.
What actually helps
Present numbers you own. Attestation rests on the originating carrier being able to say the number is yours. Buy the numbers through the provider that carries the traffic and A-level follows naturally.
Register your business name. CNAM puts your name beside the number on US handsets. A recognised name is answered; a bare number is not.
Check your own reputation. The major analytics providers each run a free lookup and a remediation form. If a number of yours is mislabelled, that form is how it gets corrected — nobody else will do it for you.
Fix the behaviour, not just the signing. If your answer rate is low because the list is poor, no amount of attestation will help. The system is measuring something real.
If you are calling the US from outside it
This is where most international traffic ends up at C-level: the call enters the US through a gateway that has no relationship with you and no way to vouch for your caller ID.
The way through is to originate with a carrier that can attest for you — one that has issued you the US numbers you present. Ask any provider quoting you US rates what attestation your calls will carry, and get the answer before you commit traffic.
Our US routes sign at A-level where the number is yours.
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