Q.850 cause 27 Destination out of order

What Q.850 cause 27 means

The far end could not be signalled at all because its interface is down — a PBX offline, a broadband outage at the site, or a gateway that has stopped answering. The route exists; the equipment at the end of it does not respond. SIP equivalent is 502 Bad Gateway.

Normal class
Sent by The switch or SBC nearest the failed destination
Occasional

What Q.850 cause 27 Destination out of order actually means

The specification defines not functioning correctly quite precisely: a signalling message could not be delivered to the remote party, because of a physical layer or data link failure at their end, or because the equipment is offline. This is a statement about the state of the destination interface, not about the number or the route. Somebody knows exactly where the call should go and cannot get a message there. RFC 3398 maps it to SIP 502 Bad Gateway, which is apt — the next hop is present in the configuration but not answering.

This cause indicates that the destination indicated by the user cannot be reached because the interface to the destination is not functioning correctly. The term "not functioning correctly" indicates that a signalling message was unable to be delivered to the remote party; e.g., a physical layer or data link layer failure at the remote party, or user equipment offline.
— ITU-T Q.850 section 6.2.7.1.21

The SIP equivalent of Q.850 cause 27

A gateway translating this cause into SIP reports it as SIP 502 Bad Gateway. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 27 on a business phone system

An on-premise PBX has lost its trunk registrationWhen the system that terminates the numbers is not registered, inbound calls have nowhere to be delivered. Power cuts, failed updates and expired credentials all produce this, and it affects every number on the trunk at once.
A broadband or circuit outage at the destination siteNo connectivity means no signalling. From the caller side it is indistinguishable from equipment failure, which is why checking the site is online should be the first step rather than the last.
The public IP address has changedWhere a carrier routes to a fixed IP rather than a registration, a router replacement or a dynamic address change breaks delivery silently. Everything looks configured correctly at both ends and nothing arrives.
A gateway or SBC that has hungEquipment can be powered, reachable by ping and still not processing call setup. This is the most confusing variant, because basic connectivity tests all pass while calls continue to fail.
Layer 1 or layer 2 down on a legacy circuitOn surviving ISDN links, a physical fault or a data link that has not come back up after an interruption produces this cause for every call offered to the circuit.

How to fix Q.850 cause 27

Check the destination site is onlineBefore touching any telephony configuration, confirm the site has internet connectivity and that the phone system is powered. A high proportion of cause 27 investigations end here.
Look at trunk registration stateOn the PBX and in the provider portal, check whether the trunk is registered and when it last succeeded. A registration that expired hours ago pins the failure to a specific time you can correlate with other events.
Check whether the public IP has changedCompare the address the carrier has configured against the address the site is actually using. This is the standard casualty of a router swap or a broadband provider change.
Restart the gateway rather than only testing connectivityWhere the device responds to ping but not to call setup, a controlled restart of the SIP service or the device itself is the quickest way to distinguish a hang from a configuration problem.
Put a failover divert in place for next timeConfigure the carrier to divert inbound calls to mobiles or another site when the trunk is unreachable. Then an outage costs you audio quality rather than every incoming call.

Fixing the underlying problem

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

Questions about Q.850 cause 27

What is the difference between cause 27 and cause 38?

Cause 27 blames the destination interface: the network is fine, the equipment at the end is not reachable. Cause 38 says the network itself is out of order. If you own the destination, cause 27 is pointing at your own kit or your own connectivity.

We are getting cause 27 on our own inbound numbers. What do we check first?

Connectivity and registration, in that order. Confirm the site is online, then confirm the trunk is registered with the carrier. Between them these explain most occurrences, and both can be checked in a couple of minutes.

How is cause 27 different from cause 18?

Cause 18 is a timeout — the sender waited and heard nothing. Cause 27 is an explicit report from a switch that already knows the destination interface is down. Cause 27 is better information, because somebody has actually diagnosed the failure rather than simply given up.

Related Q.850 cause values

Source. Q.850 cause 27 Destination out of order is defined in ITU-T Q.850 section 6.2.7.1.21 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