SIP 504 Server Time-out

What SIP 504 means

A server did reply, to tell you it could not get an answer from something further along. Unlike a 408, where nothing came back at all, a 504 confirms your signalling is reaching a working server and the problem lies beyond it.

Server error (5xx)
Sent by A server that could not get a timely answer upstream
Occasional

What SIP 504 Server Time-out actually means

The server did not receive a timely response from an external server it needed to consult. That distinction from a 408 is diagnostically valuable: your path to that server is fine, so you can stop investigating your own firewall and start looking upstream.

What causes SIP 504 on a business phone system

The carrier is not responding to the providerA route whose upstream carrier has gone quiet produces a 504 to you. Affects specific destinations more often than everything.
A DNS record points at an unreachable hostWhere service records list a host that no longer answers, the platform waits and then times out.
An upstream element is overloadedA busy element that eventually answers too late produces the same result as one that never answers.

How to fix SIP 504

Test several destinationsIf only some fail, it is an upstream route. If all fail, it is closer to the platform. This narrows it in one test.
Confirm your trunk host records are currentCompare against your provider current documentation, since host details change after platform migrations.
Report it with the affected numbers and a Call-IDA 504 is beyond your edge. Giving the provider the destinations that fail and one Call-ID gets it looked at properly.

Fixing the underlying problem

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

Firewall and network configuration for SIP trunks

Questions about SIP 504

What is the difference between SIP 408 and SIP 504?

A 408 means nothing answered you at all, so suspect your own firewall or NAT. A 504 means a server did answer, to say it could not get a reply from something further upstream. A 504 therefore proves your signalling path is working and moves the investigation beyond you.

Call hangup causes that become SIP 504

If the call crossed a gateway between SIP and ISDN or SS7, this status may have started life as one of these Q.850 cause values. The mapping is defined in RFC 3398 section 7.2.4.1.

Q.850 cause 102Recovery on timer expiry

Related SIP response codes

Source. SIP 504 Server Time-out is defined in RFC 3261 section 21.5.5 and registered in the IANA SIP Parameters — Response Codes. The causes and fixes above are drawn from our own experience supporting UK business phone systems.

All VoIP & SIP codes