How to Get Full A-Level Attestation for Your Outbound Calls

Why Most Outbound Operations Are at B-Level

The STIR/SHAKEN standard requires that the signing carrier make a specific certification: this caller has the right to use this telephone number on this call. Full attestation (A-level) requires the carrier to be able to verify that authorization.

The most common reason for B-level signing is the number ownership gap. Here is how it happens:

A call center buys SIP trunking from Provider A. They want to present 20 different caller IDs. Provider A sources those numbers from a number inventory that they manage, or that they have leased from a wholesale provider, or that the customer ported in. Provider A can authenticate the call center as a customer — they know who you are. But for numbers they do not directly control and have not directly issued to you, they cannot certify your right to use them. B-level is the technically honest answer in this case.

Getting to A-level requires closing that gap. Either the provider directly issues you the numbers (owns or directly controls them), or you have a documented authorization process the provider accepts as sufficient for certification.

The Three Paths to A-Level

Path 1: Use numbers directly issued by your SIP trunk provider. When your provider generates the DID and assigns it to your account, they have a direct chain of custody. They issued the number, they know you are their customer, and they can certify both. This is the cleanest path to A-level and requires no additional paperwork.

UnlimCall issues caller IDs on demand across 33 live markets, assigned directly from our network inventory. There is no reseller in the middle, no ported-in number with opaque history, and no shared pool. The number is yours for the duration you need it, issued by the carrier signing your calls.

Path 2: Port your numbers to your SIP trunk provider. If you own specific numbers that are integral to your brand — a toll-free number printed on physical materials, or a local number with established market presence — you can port them to a provider that can then directly manage them and issue A-level attestation. The porting process transfers number management to the signing carrier, satisfying the authorization requirement.

Porting takes time (1–5 business days depending on the losing carrier), and you will have B-level attestation during the transition window. Factor this into campaign scheduling.

Path 3: LOA-based authorization with a compatible provider. Some carriers accept a Letter of Authorization from the number's controlling entity and will use that documentation to issue A-level attestation on behalf of the number owner. This path is more common for large enterprises with national number portfolios managed by a separate telecom team. It is less common in the SMB call center space and requires a provider that has implemented the authorization workflow in their STIR/SHAKEN signing infrastructure.

What You Need to Confirm with Your Provider

Before assuming you have A-level attestation, ask your provider these specific questions:

1. For the caller IDs I am presenting on my outbound calls, what attestation level are you signing?
2. Are those numbers in your direct inventory, ported to your network, or sourced from a third-party pool?
3. Can you show me the atok value in a sample call trace for my traffic?

The third question matters because providers sometimes say "we support full attestation" when they mean "our infrastructure is capable of full attestation" — not that your specific calls are being signed at A-level. The call trace is the ground truth.

Verifying Attestation in SIP Headers

Every STIR/SHAKEN-signed call carries the attestation level in the SIP Identity header as a JSON Web Token. The relevant field is the atok claim. A value of "A" means full attestation. "B" is partial. "C" is gateway.

If you have access to SIP trace logging in your dialer platform, pull a sample of outbound call traces and check the Identity header. The JWT can be base64-decoded to read the atok value. If your platform does not expose SIP headers, ask your provider for a sample trace on a test call to a number you control.

This two-minute verification tells you definitively what level your traffic is signed at — not what your provider's documentation says, but what is actually on the wire.

The Interaction With Caller ID Rotation

If you are rotating caller IDs because of labeling pressure, you need to understand how attestation interacts with rotation. Rotating to a fresh number does not change your attestation level. If your provider can only sign your traffic at B-level, every new number you rotate to starts at B-level. You get a clean behavioral slate, which helps — but you are not improving your attestation baseline.

If you are at A-level, rotation still makes sense as a behavioral hygiene practice: fresh numbers have no complaint history. But the returns are higher at A-level because you are starting each rotation with both a clean behavioral slate and a full trust prior in the carrier scoring model.

See how attestation and answer rate interact in our answer rates breakdown.