Stop ‘Spam Likely’: STIR/SHAKEN to Verify SIP Calls

Your agents make a legitimate call, but the customer’s phone says “Spam Likely” or blocks it outright. That’s lost revenue and trust. The fix isn’t just CNAM—it’s deploying STIR/SHAKEN so downstream carriers can verify your caller identity.

What STIR/SHAKEN actually does (plain English)

• STIR signs the caller identity in SIP using a PASSporT token. Standard: RFC 8224.
• SHAKEN defines how service providers use those signatures in IP networks (policy & governance). Overview: ATIS SHAKEN framework (public overview document).
• The result: terminating carriers can check the signature and display trusted caller ID—and are less likely to label your calls as spam.

Attestation levels (why some calls still look risky)

• A (Full): You are the customer and you own the calling number.
• B (Partial): You are the customer, but number ownership can’t be confirmed.
• C (Gateway): The provider is just passing traffic.
  Aim for A wherever possible—mixing levels (A for support, C for some outbound) can still damage reputation.

Rich Call Data (RCD) helps with branding

You can attach business name, logo, and call reason inside the PASSporT (when the ecosystem supports it). Spec: RFC 8946 – PASSporT RCD. Used correctly, RCD improves answer rates without resorting to aggressive redial strategies.

Common pain points—and fixes

1) My calls show “Spam Likely”

• Fixes:
  • Use a provider that issues A-level attestation for numbers you control (port numbers if needed).
  • Warm up new CLIs gradually; avoid bursts and short, high-abandon campaigns.
  • Keep CNAM clean and consistent with your brand.

2) We don’t know how to get certificates

• Fixes:
  • Your carrier typically handles STI certificates via the industry authority (see the STI-PA governance pages).
  • If you sign locally, use an SBC that supports STIR/SHAKEN or an open library such as [libstirshaken](https://github.com/s SignalWire/libstirshaken) (open-source) to create/verify PASSporTs.

3) We use Asterisk/OSS—where do we start?

• Fixes:
  • Review Asterisk STIR/SHAKEN guidance for module options and integration patterns.
  • If your edge proxy does the signing, ensure Identity headers survive B2BUA/translation.

4) Numbers still get flagged after we sign

• Fixes:
  • Check your attestation mix; elevate B/C where possible.
  • Maintain call reputation hygiene: valid caller IDs, sane pacing, clear opt-out, low complaint rates.
  • Use RCD (where supported) to provide context (“Your delivery is arriving now”).

5) International and non-IP scenarios

• Reality: STIR/SHAKEN is strongest within IP interconnects. Legacy TDM routes can strip identity.
• Fixes: Prefer IP peering paths that preserve PASSporT; coordinate with your carrier on border behaviour.

Rollout checklist (copy to your tracker)

1. Inventory numbers; port into ownership where practical.
2. Choose a carrier/SBC path that supports A-attestation for owned CLIs.
3. Enable signing on outbound and verification on inbound.
4. Add RCD where supported; align with marketing for names/logos.
5. Monitor: verification results, reputation analytics, answer rates.
6. Establish campaign policy: pacing, retries, opt-out, quiet hours.
7. Document fallbacks for routes that drop identity (and track them).

Measuring success

• Answer rate up, spam labelling down, and verification pass rate near 100% for owned numbers.
• Support teams see fewer “Why does your number look spammy?” tickets.
• Inbound verification helps your agents avoid scams too.

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