491 4xx Client error RFC 3261
SIP 491 Request Pending
The server had a pending request in the same dialog (glare). Both sides sent re-INVITEs at the same time.
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Common causes
- Simultaneous re-INVITEs, for example both sides putting the call on hold or refreshing session timers.
How to fix it
- Usually self-healing: each side retries after a random delay. If frequent, align session-timer refreshers so only one side refreshes.
Seeing 491 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.