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SIP response codes

SIP 491 Request Pending

Request Pending: both sides tried to change the call (re-INVITE) at the same moment. Each waits a random time and retries.

491
Name
Request Pending
Class
4xx · Client error
Defined in
RFC 3261

When it happens

  • Simultaneous hold, transfer or codec change from both ends.

What to check

  • Usually harmless. If it repeats endlessly, one side is not respecting the random back-off.

What the response looks like

SIP/2.0 491 Request Pending
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
Content-Length: 0

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