Attestation Levels (A, B, C)
Attestation levels are the three trust ratings that an originating carrier assigns to outbound calls under STIR/SHAKEN. They tell the terminating carrier how confident the originator is that the calling number is legitimate.
The three levels
| Level | Means | Customer relationship | Number ownership |
|---|---|---|---|
| A, Full | Authenticated | Direct customer of the originating carrier | Carrier confirmed customer has rights to use the number |
| B, Partial | Authenticated customer, unverified number | Direct customer | Number presented but ownership not verified |
| C, Gateway | Pass-through | None, call received from another carrier | Unknown |
Practical impact
Most US mobile carriers treat B and C calls as suspect. Numbers with a sustained B/C signature pattern are added to spam labels within hours.
If you operate at scale, request A-attestation only from your carrier. DIDHub signs all originating PSTN calls A by default; resold or transit traffic is marked C as required.
How attestation is decided
- Customer authenticates to the carrier (KYC, account verification, etc.).
- Customer is authorized to use a specific telephone number range.
- For each outbound call, the carrier checks: customer authenticated? → yes & number in their authorized range? → sign A. Number not in their range? → sign B. No customer relationship? → sign C.
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