SIP 491 Request Pending

What SIP 491 means

Both ends tried to renegotiate the same call at once, so one was asked to wait. It is usually transient and self-correcting, and only worth investigating if it coincides with audio dropping during hold or transfer.

Client error (4xx)
Sent by A device already handling a renegotiation
Occasional

What SIP 491 Request Pending actually means

The request collided with another request already in progress for the same dialog, so the server asks the sender to retry shortly. It is a collision-handling mechanism rather than an error, which is why a stray one in a log means nothing.

What causes SIP 491 on a business phone system

Simultaneous renegotiation during hold or transferHold, resume and transfer all renegotiate the session. Where both ends act at once, one defers. Common and harmless.
Session timer refreshes collidingTwo systems configured to refresh at the same interval can collide repeatedly rather than once, which is when it becomes worth fixing.

How to fix SIP 491

Ignore isolated occurrencesA 491 here and there is normal protocol behaviour and needs nothing done.
Stagger session timer intervals if it repeatsWhere it recurs on the same calls, set different refresh intervals at each end so the two do not act simultaneously.

Fixing the underlying problem

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

Diagnosing dropped and disconnected calls

Questions about SIP 491

Is SIP 491 Request Pending a problem?

Rarely. It means two renegotiations of the same call crossed and one was asked to wait, which is exactly what the mechanism is for. Only look into it if it repeats on the same calls and coincides with audio problems during hold or transfer.

Related SIP response codes

Source. SIP 491 Request Pending is defined in RFC 3261 section 21.4.27 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