Q.850 cause 42 Switching equipment congestion

What Q.850 cause 42 means

The switch handling your call was under heavy load and shed the call rather than degrade every call on the box. Cause 42 is about traffic volume, not a broken component, which is why it clusters at predictable times or during a mass-calling event. If you see it every weekday at 9am, the constraint is capacity somewhere on the route.

Resource unavailable class
Sent by The congested switch itself, which names itself in the cause
Common

What Q.850 cause 42 Switching equipment congestion actually means

The equipment reporting this is stating that it is in a period of high traffic and is applying overload protection. That is an important distinction from causes 38 and 41, which both say the network is not functioning correctly: cause 42 says the network is working exactly as designed, and the design includes refusing calls before quality collapses for everyone. RFC 3398 maps it to 503 Service Unavailable, and a well-behaved 503 can carry a Retry-After header telling your equipment how long to wait.

This cause indicates that the switching equipment generating this cause is experiencing a period of high traffic.
— ITU-T Q.850 section 6.2.7.2.6

The SIP equivalent of Q.850 cause 42

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 42 on a business phone system

Genuine peak-hour load on a carrier switchVoice traffic in UK business hours is sharply peaked, with the heaviest load in mid-morning and again after lunch. A switch running close to its engineered limit will shed a small proportion of calls at those peaks.
A mass-calling eventA broadcast phone-in, a ticket release, a major weather incident or a large outage elsewhere sends call volumes far above normal. You may be an innocent bystander whose calls share a switch with the event.
Your own channel ceiling being reachedWhere a trunk is sold with a fixed number of simultaneous calls, attempts beyond that number are refused. This produces a very clean pattern: failures start at the same concurrency figure every time.
An overloaded on-premise PBX or session border controllerUnder-specified hardware, or a box also running recording and transcoding, can hit processor limits before the trunk does. The cause looks the same from the handset, but the busy element is in your comms room.
Deliberate call gapping by the carrierNetworks protect themselves by throttling calls to a destination that is being overwhelmed, allowing only a proportion through. Your calls are being refused on purpose, and no amount of retrying changes the ratio.

How to fix Q.850 cause 42

Work out whose ceiling you are hittingCompare the number of calls in progress at the moment of failure with the channel count you pay for. If the two match, the answer is your trunk size and no carrier ticket is warranted.
Pull concurrency figures rather than call totalsDaily call volume tells you almost nothing here; simultaneous calls at the busiest minute tells you everything. Most platforms can report peak concurrency, and that single number sizes the trunk properly.
Add channels or move to a bursting arrangementIf the peak is real and recurring, buying for the peak is the fix. Some providers offer trunks that flex above the committed number for a per-call charge, which suits a business with rare but severe peaks.
Pace outbound campaigns instead of launching them at onceA dialler or a bulk outbound push can create your own congestion. Spreading the same volume over a longer window removes the peak without reducing the work done.
Ask the carrier whether gapping was appliedIf your concurrency was well below the limit and the failures hit one destination range, ask directly. Carriers can usually confirm whether a restriction was in force and roughly how long it lasted.

Fixing the underlying problem

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

Questions about Q.850 cause 42

Does cause 42 mean I need more SIP channels?

Only if the congestion is on your own trunk, which you can check by comparing simultaneous calls at the time of failure against your channel allowance. If you were nowhere near the limit, the busy equipment is the carrier's and buying more channels will not change anything.

Why does cause 42 only happen at certain times?

Because it is a load code by definition. Congestion appears when demand meets a limit, so it tracks the working day, campaign schedules and unusual events rather than appearing randomly the way an equipment fault would.

Is it worth redialling after cause 42?

A single retry is reasonable, since load fluctuates second by second. Automatic rapid retries by a dialler or PBX are counterproductive, because they add to the very load causing the refusals.

Related Q.850 cause values

Source. Q.850 cause 42 Switching equipment congestion is defined in ITU-T Q.850 section 6.2.7.2.6 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