Q.850 cause 57 Bearer capability not authorized

What Q.850 cause 57 means

The call asked for a type of connection, called a bearer capability, that the equipment supports but this line is not permitted to use. Cause 57 is about permission rather than capability: ordinary speech may be fine while a digital data bearer is refused. It maps to SIP 403 Forbidden and is uncommon outside ISDN interworking.

Service or option unavailable class
Sent by The exchange or gateway that checks subscription rights
Rare

What Q.850 cause 57 Bearer capability not authorized actually means

Every ISDN call setup carries a bearer capability describing what the connection is for, with speech, 3.1 kHz audio and unrestricted digital information being the ones you meet in practice. Speech allows the network to compress and process the audio; 3.1 kHz audio asks it to preserve the full audio band for modems and fax; unrestricted digital information asks for a clear data path. Cause 57 says the requested bearer is available on that equipment but the subscription does not include it, and RFC 3398 maps it to 403 Forbidden. The refusal comes from a subscription check, so no amount of retrying changes the outcome.

This cause indicates that the user has requested a bearer capability which is implemented by the equipment which generated this cause but the user is not authorized to use.
— ITU-T Q.850 section 6.2.7.3.5

The SIP equivalent of Q.850 cause 57

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 57 on a business phone system

A data bearer requested on a voice-only subscriptionDigital bearer services were sold separately from telephony and most modern accounts do not include them. A device that requests one gets refused even though the network is perfectly capable of providing it to somebody else.
A legacy PBX signalling the wrong bearer for voice callsOlder systems can be configured to signal unrestricted digital information for calls that are really speech. The call is refused for lack of authorisation when nothing about the call needed a data bearer in the first place.
A gateway presenting a bearer the trunk is not entitled toInterworking equipment translating between protocols may derive a bearer capability from what it received. If it derives one outside the trunk's authorisation, calls fail in a way that looks unrelated to the original SIP request.
A service ordered but never fully provisionedWhere a data or video service was ordered, the authorisation record may not have been completed even though the capability is present. This produces a clean, consistent refusal from the first attempt onwards.

How to fix Q.850 cause 57

Capture which bearer capability was actually requestedA trace of the call setup shows the bearer capability element and its value. Without that, everything else is guesswork, and the value very often reveals the problem immediately.
Compare it against what the account is authorised forAsk the carrier which bearer services the line may use. A mismatch confirms the diagnosis and turns the ticket into a provisioning request.
Correct the originating system if the bearer is wrongVoice calls should signal speech or 3.1 kHz audio, not a digital bearer. Fixing the originating configuration is usually cheaper and faster than buying an authorisation you do not need.
Only request authorisation if the service is genuinely requiredAsk what the application actually is before ordering a bearer service. In most surviving cases the underlying need can be met over IP without an ISDN-era bearer at all.

Fixing the underlying problem

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

Questions about Q.850 cause 57

What is a bearer capability?

It is the part of an ISDN call setup that says what kind of traffic the connection will carry, such as speech, 3.1 kHz audio for fax and modems, or a clear digital path. The network uses it to decide how to treat the call and whether the subscriber is allowed to ask for it.

What is the difference between cause 57 and cause 58?

Cause 57 is a permission refusal that will happen every time until the subscription changes. Cause 58 means you are entitled to that bearer but it is not free at the moment, so the same call may well succeed shortly afterwards.

How does cause 57 differ from cause 65?

Cause 65 means the equipment does not support that bearer at all, so no subscription change will help. Cause 57 means it does support it and is refusing you specifically, which is a commercial matter rather than a technical limit.

Related Q.850 cause values

Source. Q.850 cause 57 Bearer capability not authorized is defined in ITU-T Q.850 section 6.2.7.3.5 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