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Glossary · Numbering

What is Call blocking?

  • Also called: blocked calls, carrier call blocking

Reviewed by Sujan ThapaliyaLast updated

Definition

Call blocking is a carrier or handset refusing a call outright rather than labelling it. Blocked calls never ring, and the caller usually hears normal ringing or a generic failure, which is why blocking is often invisible to the business doing the calling.
The three STIR/SHAKEN attestation levels, which underpin how call blocking behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, call blocking included.

In practice

How call blocking actually works

Carrier-level blocking is applied by analytics providers on behalf of terminating networks, using the same scores that produce spam labels. A number scored badly enough moves from labelled to blocked.

The invisibility is the operational problem: answer rate collapses while dial counts and dispositions look normal, because a blocked call often registers as unanswered rather than failed.

Remediation follows the same path as label removal — evidence, registration, and a change in the calling pattern that produced the score.

Blocking is also increasingly automatic rather than requested. Carriers now block or divert traffic that scores badly without the recipient asking, under regulatory pressure to reduce unwanted calls, which means a business can lose reachability to an entire network without a single customer complaining to anyone.

Worked example

Blocked calls often register as unanswered rather than failed, so dial counts and dispositions look normal while the answer rate quietly collapses. That is what makes blocking harder to spot than labelling.

Call blocking: common questions

How do I know if my number is being blocked?
Test from handsets on each major network. A number that rings normally from your own line and never rings on another carrier is being blocked or labelled at that carrier, not failing generally.
Can a blocked number be unblocked?
Usually, through the analytics providers' remediation processes. It requires evidence of who you are and a corrected calling pattern; submitting without changing the pattern produces a temporary result.
Is blocking the same as being labelled Spam Likely?
No, it is the next step. Labelling still rings the phone with a warning; blocking means the call never arrives at all.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See call blocking in the product

Branded caller ID puts your business name on the recipient's screen instead of a bare number. In North America it works through CNAM, the caller-name record attached to a number, combined with STIR/SHAKEN signing that lets the terminating carrier trust the call is really from you.

Explore branded caller idGet early access