Q.850 cause 29 Facility rejected

What Q.850 cause 29 means

A supplementary service was requested and the network refused to provide it. The call itself may be fine; it is the feature — a transfer, a diversion, a CLI presentation request — that was turned down. Common after an ISDN to SIP migration when a feature is no longer carried. SIP maps it to 501 Not Implemented.

Normal class
Sent by The network or far-end switch that was asked to provide the service
Rare

What Q.850 cause 29 Facility rejected actually means

Supplementary services are the extras layered on top of a basic call: transfer, diversion, hold, conference, number presentation, message waiting, advice of charge. This cause is returned when the user asked for one of them and the network cannot supply it. Importantly, it says nothing about the call being invalid — the same call would often work fine without the feature request attached. RFC 3398 maps it to SIP 501 Not Implemented, which matches the meaning closely: the request was understood and the capability is not there.

This cause is returned when a supplementary service requested by the user cannot be provided by the network.
— ITU-T Q.850 section 6.2.7.1.23

The SIP equivalent of Q.850 cause 29

A gateway translating this cause into SIP reports it as SIP 501 Not Implemented. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 29 on a business phone system

A transfer method the trunk does not supportMany carriers do not accept SIP REFER from customer equipment, because it hands call control outward. A PBX configured to transfer that way will have the request refused while transfers handled internally work fine.
A CLI presentation request that is not provisionedAsking to present a specific number, or to withhold the number, requires the service to be enabled on the trunk. Where it is not, the request is rejected rather than silently ignored.
A feature that did not survive the ISDN migrationNetwork-level diversion, advice of charge and similar ISDN services do not always have a direct equivalent on a SIP trunk. A PBX configuration carried over unchanged keeps asking for something that no longer exists.
Message waiting indication on a trunk that does not carry itWhere voicemail sits on one side of a trunk and the phones on the other, the notification has to cross it. If the trunk does not support the mechanism, the request comes back rejected.
Network hold or conference invoked from the wrong sideFeatures that the PBX should handle locally are sometimes requested from the network by default. The network declines, and the fix is to move the feature inward rather than to have it enabled.

How to fix Q.850 cause 29

Work out which feature triggered itCorrelate the failures with what users were doing — transferring, diverting, putting a call on hold. The diagnostic field frequently names the service, which removes the guesswork entirely.
Handle the feature on the PBX instead of the networkMost supplementary services can be performed locally. Configuring transfers to be completed by the PBX rather than passed to the carrier resolves the majority of these cases without needing anything enabled.
Ask the carrier what the trunk actually supportsGet a written list of supported supplementary services for your trunk product. Compare it against what your PBX is configured to request, and remove or re-home anything that is not on the list.
Re-check the feature list after any migrationWhenever you move from ISDN to SIP, or between carriers, walk through transfers, diverts, hold, conference and presentation deliberately. Features fail quietly and users work around them for months before anybody reports it.

Fixing the underlying problem

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

Questions about Q.850 cause 29

What is the difference between cause 29 and cause 50?

Cause 29 says the service cannot be provided. Cause 50 says the service exists but you have not subscribed to it. In practice the first often means it is not available on the platform, and the second means it is available and could be added to your account.

Transfers stopped working after we moved to SIP and we see cause 29. Why?

Your PBX is most likely asking the carrier to complete the transfer, and the carrier does not allow it. Reconfigure it to handle transfers itself so the audio path is rebuilt through your own system rather than handed to the network.

Related Q.850 cause values

Source. Q.850 cause 29 Facility rejected is defined in ITU-T Q.850 section 6.2.7.1.23 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