Q.850 cause 88 Incompatible destination

What Q.850 cause 88 means

The destination was reached but could not accept the kind of call being offered, because compatibility details such as low layer, high layer or data rate did not match. Cause 88 is a mismatch between endpoints rather than a network failure, which is why it shows up on fax machines, alarm diallers and card terminals. It maps to SIP 503 Service Unavailable.

Invalid message class
Sent by The called equipment or the switch serving it
Rare

What Q.850 cause 88 Incompatible destination actually means

Alongside the bearer capability, a call setup can carry low layer and high layer compatibility information describing the terminal the call is meant for. On an old ISDN line with a fax, a phone and a card terminal sharing one number range, this is what let each device answer only the calls intended for it. Cause 88 means the far end examined those attributes and concluded it could not accommodate the call, so the call reached the right place and was turned away on the doorstep. RFC 3398 maps it to 503 Service Unavailable, which is misleading in tone, because nothing here is congested or broken.

This cause indicates that the equipment sending this cause has received a request to establish a call which has low layer compatibility, high layer compatibility, or other compatibility attributes (e.g., data rate) which cannot be accommodated.
— ITU-T Q.850 section 6.2.7.5.8

The SIP equivalent of Q.850 cause 88

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 88 on a business phone system

A voice call reaching a fax, alarm or data lineThe number answers, but the device behind it only accepts a specific kind of call. Ringing it from a handset will fail in exactly this way while the device works normally for its intended traffic.
Fax or modem tones crossing a compressed codecCompression designed for speech destroys the tones fax and modems rely on. The negotiation fails, and depending where it fails you may see this cause rather than a media error.
Alarm panels and card terminals moved onto VoIPDevices that dialled reliably over an analogue or ISDN line frequently misbehave once the path is IP. This is a very real issue in UK businesses working through the PSTN switch-off, and it usually needs a designed replacement rather than a tweak.
A call placed to the wrong sub-address on a multi-terminal lineWhere several devices share a line and are distinguished by compatibility information, dialling the wrong one reaches equipment that will not take the call. It is a numbering error presenting as a technical fault.
A video or wideband call offered to a voice-only endpointThe destination may support speech and nothing more. The refusal is a straightforward statement of what the terminal is, not evidence of a problem in the path.

How to fix Q.850 cause 88

Find out what device actually answers that numberBefore touching any configuration, establish whether the destination is a phone, a fax, an alarm or a payment terminal. Half of these cases end here, with a number that was never meant to take a voice call.
Test with an ordinary voice call from a mobileA plain call from outside your system separates a destination problem from a configuration problem at your end. If the mobile fails too, stop looking at your PBX.
Give fax and data devices a purpose-built pathUse T.38 where it is supported, or move the function to email or an IP service. Forcing a legacy device down a voice path tends to produce intermittent success, which is worse than an outright failure because it hides the problem.
Check the analogue adaptor settings behind legacy devicesAn ATA determines how an old device is presented to the network, including codec and fax handling. Getting these right resolves many alarm and terminal problems without changing the device itself.
Audit legacy dialling devices ahead of the switch-offLift attendants, alarm panels, card machines and door entry systems all dial out and all need checking. Doing this as a planned exercise costs far less than discovering it device by device after a migration.

Fixing the underlying problem

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

Questions about Q.850 cause 88

Why do fax calls return cause 88?

Because a fax machine and a voice path want different things from the connection, and the far end says so. Getting the call to connect is only half the job, since fax over a compressed codec fails in ways that are harder to diagnose than a clean refusal.

Is cause 88 a fault on my line?

Usually not. Your line delivered the call to the destination and the destination declined the type of call, so the investigation belongs at the far end or with what your equipment is offering.

Does cause 88 affect alarms and card machines?

It can, and those devices deserve attention in their own right during a move to VoIP. They dial infrequently, so a failure can go unnoticed for months until the one occasion it matters.

Related Q.850 cause values

Source. Q.850 cause 88 Incompatible destination is defined in ITU-T Q.850 section 6.2.7.5.8 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