Q.850 cause 34 No circuit/channel available

What Q.850 cause 34 means

There was no free circuit to carry the call. On a SIP trunk that normally means every contracted channel was in use; on a carrier route it means congestion further out. It is a capacity answer, not a fault with any phone or number. Gateways report it to SIP as 503 Service Unavailable.

Resource unavailable class
Sent by The carrier, or your own PBX when it runs out of capacity
Common

What Q.850 cause 34 No circuit/channel available actually means

This sits in the resource unavailable class rather than the normal class, and that classification is the point: the call was perfectly valid and there was simply nothing free to carry it. In a modern UK business the circuit is a licensed concurrent call on a SIP trunk rather than a physical timeslot, but the arithmetic is the same — you have a ceiling, and calls above it are refused. RFC 3398 maps it to SIP 503 Service Unavailable. If it appears at predictable times of day, you are looking at a sizing question rather than a fault.

This cause indicates that there is no appropriate circuit/channel presently available to handle the call.
— ITU-T Q.850 section 6.2.7.2.1

The SIP equivalent of Q.850 cause 34

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 34 on a business phone system

The trunk channel limit is reached at peakSIP trunks are sold with a set number of simultaneous calls. Monday mornings and the hour after a marketing email land are when businesses discover the number they bought was based on optimism.
A PBX concurrent call licence ceilingSeparate from the carrier limit, the phone system itself may be licensed for a maximum. Calls can be refused with capacity still available on the trunk, which makes the arithmetic confusing until you check both.
Congestion on a specific carrier routeIf only calls to one destination or country fail while everything else works, the shortage is on the far side rather than on your trunk. Common on international routes and during large-scale events.
A partially failed trunk with reduced capacityWhere a trunk group is delivered over more than one path and one path is down, the effective ceiling halves without anything appearing to be broken. Calls fail at half the volume you would expect.
Toll fraud consuming your channelsFraudulent traffic occupies capacity like any other call. Legitimate users being refused at odd hours, particularly overnight or at a weekend, deserves an urgent look at outbound call records.

How to fix Q.850 cause 34

Count concurrent calls at the moment of failurePull the CDR for the failing period and count how many calls were up simultaneously. If it equals your contracted channel count, the diagnosis is complete and the rest is commercial.
Establish whether it is all calls or one destinationIf UK calls succeed while one country fails, the constraint is a carrier route rather than your capacity, and no amount of extra channels will help. Raise it with the carrier as a route congestion issue.
Check both the carrier and PBX limitsThese are two independent ceilings and either can be the binding one. Write down both numbers and compare them against your observed peak before ordering anything.
Review outbound records for fraudBefore buying more capacity, confirm the traffic is yours. Look for unfamiliar destinations, calls outside working hours and repeated attempts to the same international number.
Add capacity or an overflow routeEither increase the contracted channels, or configure inbound overflow to mobiles and a message so callers are not simply refused. Many carriers offer burst capacity, which suits businesses with sharp seasonal peaks.

Fixing the underlying problem

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

Questions about Q.850 cause 34

Does cause 34 mean I need more SIP channels?

Often, but check before you buy. Count the simultaneous calls at the time of failure and compare with your contracted number. If they match, more channels is the answer; if they do not, the shortage is somewhere else and extra capacity will not help.

What is the difference between cause 34 and cause 17?

Cause 17 means the person you called was occupied. Cause 34 means there was no circuit available to carry the call in the first place, so nobody was ever contacted. One is about a phone, the other about capacity.

We only get cause 34 on international calls. Why?

That pattern points at congestion on a specific carrier route rather than at your trunk. If UK calls are connecting normally at the same moment, your capacity is fine and the problem is further out. Report it to your carrier with example destinations and times.

Related Q.850 cause values

Source. Q.850 cause 34 No circuit/channel available is defined in ITU-T Q.850 section 6.2.7.2.1 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