Q.850 cause 58 Bearer capability not presently available

What Q.850 cause 58 means

Everything about this call was permitted; the resource needed to carry it simply was not free at that moment. Cause 58 sits between cause 57, which is a permissions refusal, and cause 65, which is a flat statement of no support. Because the shortage is temporary, the same call often succeeds shortly afterwards. It maps to SIP 503 Service Unavailable.

Service or option unavailable class
Sent by The switch or gateway that owns the requested bearer resource
Rare

What Q.850 cause 58 Bearer capability not presently available actually means

This is the only bearer-related cause in the service class that behaves like congestion. The equipment implements the requested bearer, the subscriber is entitled to it, and it happens to be fully committed right now. That makes it a capacity question rather than a configuration one, and it is why RFC 3398 maps it to 503 Service Unavailable alongside the congestion causes, rather than to the 403 Forbidden it gives cause 57 and the other permission refusals. In modern equipment the scarce resource is very often a pool of digital signal processors doing transcoding.

This cause indicates that the user has requested a bearer capability which is implemented by the equipment which generated this cause but which is not available at this time.
— ITU-T Q.850 section 6.2.7.3.6

The SIP equivalent of Q.850 cause 58

A gateway translating this cause into SIP reports it as SIP 503 Service Unavailable. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 58 on a business phone system

Transcoding capacity exhausted on a gatewayConverting between codecs consumes processing resource from a finite pool, quite separately from call channels. When the pool is full, calls needing conversion are refused while calls that do not need it carry on connecting.
All channels for one bearer type occupiedWhere a platform reserves capacity per bearer type, one type can be full while the interface as a whole is quiet. The trunk looks half idle, which makes the refusal seem inexplicable until you look at the right counter.
Media server or announcement capacity used upRecording, conferencing and announcement functions draw on shared media resources. Heavy use of one of them can starve ordinary calls that need the same hardware.
A temporary carrier restriction on a bearer serviceDuring maintenance or partial failure, a carrier may keep a service authorised but unavailable rather than removing it. The distinction matters because it signals the outage is deliberate and bounded.

How to fix Q.850 cause 58

Retry to confirm the shortage is transientIf a second attempt a minute later connects, you have confirmed a capacity issue rather than a configuration one. That distinction alone changes where you look next.
Check transcoding and DSP utilisation at the time of failureMost gateways expose a counter for allocated versus available processing resource. Peaks in that counter matching the failures make the diagnosis conclusive.
Remove the need for conversion where you canAgreeing a single codec end to end, typically G.711 A-law in the UK, avoids transcoding entirely. Fewer conversions means fewer of these refusals and better audio quality as a side effect.
Size the resource pool for the busiest minuteAverages will mislead you here, because the pool empties at the peak and nowhere else. Provision against peak concurrency with headroom rather than against typical usage.

Fixing the underlying problem

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

Questions about Q.850 cause 58

Will retrying after cause 58 work?

Frequently yes, because the cause specifically means the capability is unavailable at this time rather than unavailable to you. If retries never succeed, the diagnosis is wrong and you should look at cause 57 or cause 65 instead.

Is cause 58 a codec problem?

It can be, when the scarce resource is transcoding capacity, though it is a shortage of conversion capacity rather than a failure to agree a codec. A genuine failure to agree usually appears as SIP 488 with cause 65 behind it.

Does cause 58 mean I need a bigger trunk?

Not necessarily, since the constraint is a bearer or media resource rather than the number of call paths. Adding channels without adding the underlying processing capacity often makes no difference at all.

Related Q.850 cause values

Source. Q.850 cause 58 Bearer capability not presently available is defined in ITU-T Q.850 section 6.2.7.3.6 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