Q.850 cause 21 Call rejected

What Q.850 cause 21 means

Something deliberately refused the call. The far end was neither busy nor incompatible; it could have accepted and chose not to. Usually a block list, an anonymous-caller rule, call barring, or a carrier rejecting your outbound CLI. Gateways report it as SIP 403 Forbidden.

Normal class
Sent by The called equipment, or a network policy applying a service constraint
Occasional

What Q.850 cause 21 Call rejected actually means

The specification is unusually pointed about this one: the equipment sending the cause does not wish to accept the call, although it could have. That rules out busy and rules out incompatibility, leaving a deliberate decision. The network can also generate it where a supplementary service constraint has cleared the call, and the diagnostic field may carry detail about which service and why. RFC 3398 maps it to SIP 403 Forbidden, which captures the flavour well — this is a refusal on policy grounds, not a failure.

This cause indicates that the equipment sending this cause does not wish to accept this call, although it could have accepted the call because the equipment sending this cause is neither busy nor incompatible. This cause may also be generated by the network, indicating that the call was cleared due to a supplementary service constraint. The diagnostic field may contain additional information about the supplementary service and reason for rejection.
— ITU-T Q.850 section 6.2.7.1.16

The SIP equivalent of Q.850 cause 21

A gateway translating this cause into SIP reports it as SIP 403 Forbidden. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 21 on a business phone system

The presented CLI was not acceptedUK carriers increasingly reject outbound calls whose presentation number is not valid, not allocated, or not demonstrably yours. This is the fastest-growing reason UK businesses meet cause 21, and it appears suddenly when rules tighten.
Anonymous caller rejection at the far endWhere the called party blocks withheld numbers, a call presenting no CLI is refused outright. If your system is withholding by accident, every such destination will reject you while others connect normally.
A block list entry on the destinationIndividual numbers can be barred at extension, group or account level. The refusal is instant and specific, so the caller experiences a very short call attempt that never rings.
Call barring or class of service on your own systemPremium rate, international or mobile barring applied to an extension produces a rejection before the call leaves. Check your own restrictions before assuming somebody else refused you.
A supplementary service constraint in the networkDiversion loops, closed user group rules and similar controls clear calls with this cause. The diagnostic field is the only reliable way to work out which service was responsible.

How to fix Q.850 cause 21

Verify the outbound CLI you are presentingConfirm the presentation number is a real, allocated number that you own and that your carrier has authorised for use. Fixing an invalid CLI resolves most outbound cause 21 clusters immediately.
Check whether the CLI is being withheld unintentionallyRing a mobile and see what displays. If nothing shows, a withhold setting or a missing presentation number is triggering anonymous rejection at destinations that block it.
Review block lists and class of serviceLook at both ends where you control them: destination block lists, and outbound barring on the calling extension. Both produce identical cause values but have entirely different owners.
Read the diagnostic field before escalatingWhere the network generated the cause, the diagnostic often names the supplementary service involved. Quoting it to your carrier turns a vague fault report into a specific question.
Ask the carrier why, with an example callGive them the date, time, calling and called numbers and the Call-ID. Carriers can see their own rejection reason, and for CLI-related refusals they will usually tell you exactly which check failed.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about Q.850 cause 21

Why has our carrier started rejecting outbound calls with cause 21?

Almost always the presentation CLI. If the number you send as your caller ID is not allocated to you, or is not in a valid format, carriers will refuse the call rather than pass it on. Check every outbound route sends a number you genuinely own.

What is the difference between cause 21 and cause 17?

Cause 17 means the destination was occupied and physically could not take the call. Cause 21 means it could have taken the call and decided not to. One is a capacity condition, the other is a policy decision.

How does cause 21 differ from cause 26?

Cause 21 is an active refusal by the destination. Cause 26 is not a refusal at all — it means the call was offered to several endpoints and somebody else answered it first, so this one was never awarded the call.

Related Q.850 cause values

Source. Q.850 cause 21 Call rejected is defined in ITU-T Q.850 section 6.2.7.1.16 and registered in the ITU-T Recommendation Q.850 (10/2018). The causes and fixes above are drawn from our own experience supporting UK business phone systems.

All Call hangup causes (Q.850) codes