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Glossary · Voice technology

What is Unconditional call forwarding?

  • Also called: CFU, call forward unconditional, call forwarding unconditionally

Reviewed by Sujan ThapaliyaLast updated

Definition

Unconditional call forwarding sends every incoming call to another number immediately, without ringing the original handset. It contrasts with conditional forwarding, which only redirects when the line is busy, unanswered, or unreachable.
The three STIR/SHAKEN attestation levels, which underpin how unconditional call forwarding 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, unconditional call forwarding included.

In practice

How unconditional call forwarding actually works

On GSM networks it is set with the USSD string **21*number# and cancelled with ##21#. Conditional forwarding uses **61# for no-answer, **67# for busy, and **62# for unreachable.

The message 'call forwarding unconditional - voice not forwarded' usually means the network accepted the instruction for some services but not for voice, most often because the target number is not permitted or the feature is not provisioned on the line.

Because unconditional forwarding never rings the handset, it is also the mechanism behind several account-takeover attacks. Check your forwarding status if calls stop arriving.

Unconditional forwarding is also the setting most often left behind by a departing employee. The line rings nowhere useful, the replacement cannot work out why their phone is silent, and nothing in the handset explains it — which is why clearing forwarding belongs on the offboarding checklist next to the password reset.

Worked example

Set: **21*15551234567# then call. Cancel: ##21#. Check status: *#21#.

Unconditional call forwarding: common questions

What does 'call forwarding unconditional' mean on my phone?
That a rule is set to send every incoming call to another number without ringing your handset first. It is the network reporting its configuration, and it stays set until it is explicitly cleared.
Why does it say unconditional forwarding is active but voice is not forwarded?
The network is describing a rule that exists but is not currently moving voice calls — often because the rule was set for a different service, or set and partially cleared. Clearing forwarding and setting it again from scratch resolves most cases.
How do I turn unconditional forwarding off?
On most US carriers, dial *73 and wait for the confirmation tone. On GSM networks ##002# clears every forwarding rule at once, including the conditional ones that are easy to forget are still set.

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 unconditional call forwarding in the product

An IVR (interactive voice response) system answers inbound calls and routes them using a menu, a spoken request, or data it already holds about the caller. Good IVR gets someone to the right person in one step. Bad IVR is the reason people press 0 repeatedly.

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