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SIP response codes

SIP 408 Request Timeout

Request Timeout: no final answer arrived in time. The far end or something on the way never replied.

408

Name: Request Timeout
Class: 4xx · Client error
Defined in: RFC 3261
Q.850 cause towards the PSTN: 102 · Recovery on timer expiry
Produced from Q.850 causes: 18

When it happens

The called phone is unreachable: switched off, lost its registration, or behind a NAT whose pinhole has closed.
A firewall drops SIP towards the next hop.
The next hop is overloaded or down.

What to check

Check that the called device is registered and keeps its NAT binding open (registration interval or keep-alives shorter than the router’s UDP timeout).
Trace the INVITE hop by hop to see where it stops.

What the response looks like

SIP/2.0 408 Request Timeout
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK776asdhds
From: <sip:101@pbx.example>;tag=1928301774
To: <sip:+15551234567@pbx.example>;tag=a6c85cf
Call-ID: a84b4c76e66710@203.0.113.10
CSeq: 2 INVITE
Reason: Q.850;cause=102;text="Recovery on timer expiry"
Content-Length: 0