SIP 408 Request Timeout

What SIP 408 means

No reply arrived before the timer expired. A 408 is a statement that signalling went unanswered, so it points at the network path rather than at the destination: a firewall dropping SIP, a closed NAT pinhole, or SIP ALG interfering.

Client error (4xx)
Sent by Your own device or PBX, when nothing answered in time
Common

What SIP 408 Request Timeout actually means

The server could not produce a final response within a reasonable time, so the transaction was abandoned. Because SIP retransmits before giving up, a 408 means several attempts went unanswered, which is why it so reliably indicates a one-way or broken signalling path rather than a busy or unavailable destination.

What causes SIP 408 on a business phone system

A firewall is allowing SIP out but not the replies backAsymmetric firewall rules are the classic cause. Requests leave, responses are dropped, and every transaction eventually times out. Frequently appears immediately after a firewall change or a new router.
The NAT pinhole closed before the reply arrivedWhere the registration or keep-alive interval is longer than the router UDP timeout, the return path is closed by the time the far end responds. Inbound calls suffer far more than outbound ones.
SIP ALG has rewritten the message so replies go nowhereA router rewriting SIP headers can substitute an address that the far end cannot reach, so the response is sent somewhere that will never receive it.
The far end is genuinely not respondingA carrier or destination PBX that is down produces the same timeout. Worth ruling in or out early by testing a different destination.

How to fix SIP 408

Disable SIP ALG on the router or firewallDo this before anything more involved. It is enabled by default on many consumer routers, it breaks SIP in ways that look like a dozen other faults, and switching it off costs nothing.
Check that SIP replies are allowed back inConfirm the firewall permits return traffic on the SIP port, normally UDP 5060, from your provider. Look specifically for a rule that permits outbound without a matching stateful return path.
Shorten the registration or keep-alive intervalBring it below the router UDP timeout, commonly 120 to 180 seconds, so the return path is held open.
Try TCP or TLS transportWhere UDP keeps timing out through awkward network equipment, a connection-oriented transport often sidesteps the problem entirely, and TLS has the side benefit of stopping ALG interfering at all.

Fixing the underlying problem

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

Questions about SIP 408

What does SIP 408 Request Timeout mean?

Your device sent a SIP request, retransmitted it, and never got a reply, so it gave up. It is a network path problem rather than a problem with the person you called: something between you and the far end is dropping or misdirecting the signalling.

What is the difference between SIP 408 and SIP 504?

Who ran out of patience. A 408 is your own transaction timing out because nothing came back at all. A 504 is a server that did reply, telling you it could not get an answer from something further upstream. A 408 means silence; a 504 means someone answered to say they were also waiting.

Why do I get 408 only on incoming calls?

Because inbound calls depend on your provider being able to reach through your router, while outbound calls open the path themselves. A registration interval longer than your router NAT timeout, or a firewall that permits SIP out but not back in, produces exactly this asymmetry.

Call hangup causes that become SIP 408

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 18No user responding

Related SIP response codes

Source. SIP 408 Request Timeout is defined in RFC 3261 section 21.4.9 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.

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