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Glossary · Voice technology

What is Anonymous call rejection?

  • Also called: acr, reject withheld calls

Reviewed by Sujan ThapaliyaLast updated

Definition

Anonymous call rejection automatically refuses incoming calls that have withheld their caller ID, playing the caller an announcement instead of ringing the line. It is a per-line carrier feature, distinct from blocking specific numbers.
The three STIR/SHAKEN attestation levels, which underpin how anonymous call rejection 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, anonymous call rejection included.

In practice

How anonymous call rejection actually works

It is effective against the subset of nuisance calling that withholds identity, and useless against the larger share that presents a spoofed or rotating number. Most unwanted calling now presents a number, so the feature solves a shrinking part of the problem.

The cost is silent: legitimate callers who withhold by default get refused with no record on your side. Switchboards, clinics and some public-sector lines withhold as standard.

On a business phone system the better pattern is to route withheld calls somewhere rather than refuse them — to an auto attendant or an AI receptionist that asks who is calling.

Worked example

It blocks the withheld calls and also the clinic confirming an appointment, the recruiter calling about an application, and the switchboard that withholds by default. Check what else it catches before enabling it.

Anonymous call rejection: common questions

Does anonymous call rejection block spam calls?
Only the ones that withhold their number. Most unwanted calling now presents a number, often a spoofed or rotating one, so the feature misses the majority of it.
What does the caller hear?
A carrier announcement telling them the line does not accept withheld calls and to unblock their number and try again. They are not put through to voicemail.
Is there a better option for a business line?
Usually yes: route withheld calls to an auto attendant or a receptionist that asks who is calling. You keep the legitimate ones and still filter the rest.

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 anonymous call rejection in the product

An auto attendant answers inbound calls with a recorded menu and sends each caller where they need to go, by keypress or by what they say. It is the front door of a phone system: no queueing, no agents, just directing traffic before anyone is involved.

Explore auto attendantGet early access